Urinary phytoestrogens were associated with reduced sensitivity to thyroid hormone in NHANES 2007-1010: A large cross-sectional study

doi:10.21203/rs.3.rs-4917925/v1

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Urinary phytoestrogens were associated with reduced sensitivity to thyroid hormone in NHANES 2007-1010: A large cross-sectional study

https://doi.org/10.21203/rs.3.rs-4917925/v1

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Background

Estrogen and thyroid hormone interact extensively in vivo, but the relationship between phytoestrogens and thyroid function and thyroid hormone sensitivity remains unclear.

Methods

Using 2007–2010 NHANES data of United States, we analysed the association of 6 types of urinary phytoestrogens with thyroid function and thyroid hormone sensitivity using a multivariate linear regression model.

Results

A total of 2,521 participants were included in the final analysis. After adjustment for multiple factors, we found that urinary phytoestrogens were associated with thyroid function and thyroid hormone sensitivity, and the associations were sex and type specific. For thyroid function: Phytoestrogens were negatively associated with FT4 and TT4 in overall analysis. After sex stratification, Equol, Enterodiol and TT4 were negatively associated in men. In women, Daidzein, O-Desmethylangolensin (O-DMA) and Genistein were positively associated with FT4, while Enterodiol was negatively associated with FT4. In addition, Enterolactone was negatively associated with TT4. For thyroid hormone sensitivity: Phytoestrogens were associated with Thyroid feedback quantile-based index (TFQI) in overall analysis. After sex stratification, Genistein was positively associated with FT3/FT4 ratio and Enterolactone was positively associated with TFQI in men. In women, Daidzein and O-DMA were negatively associated with FT3/FT4 ratio and positively associated with TFQI.

Conclusion

Urinary phytoestrogens were associated with FT4 and TT4 levels and may adversely affect the central thyroid hormone sensitivity. Given the risk of hypothyroidism and SCH associated with reduced thyroid hormone sensitivity, more health monitoring of people with high phytoestrogens intake is warranted.

Urinary phytoestrogen

Thyroid function

Thyroid hormone sensitivity

Thyroid hormone receptor

Hypothyroidism

With the deepening of thyroid related research, a new concept of "reduced sensitivity to thyroid hormone (RSTH)" has been widely accepted and adopted [1]. RSTH, also known as 'resistance to thyroid hormone', has a serious impact on people' health. Sun Y et al. [2] showed that impaired thyroid hormone sensitivity in sub-clinical hypothyroidism (SCH) was associated with increased risk of hyperuricemia and cardiovascular disease (CVD) and decreased risk of obesity. Recently, a longitudinal cohort study of individuals with mutations in the thyroid hormone receptor β gene (RTHβ) in the UK demonstrated a reduction in survival and an increase in cardiovascular morbidity in a group with impaired thyroid hormone sensitivity [3].

Whether in pursuit of a healthier lifestyle or for animal welfare, there is a growing interest in 'vegetarianism' around the world [4]. In order to prevent the potential risk of malnutrition, there has been a significant increase in the consumption of soy products and various food supplements. People tend to focus on the health benefits of soy products and supplements, but ignore the potential health risks of excessive phytoestrogen intake.

There are many interactions between different hormones in the human body. Similarly, oestrogen has a variety of regulatory effects on thyroid function. Changes in thyroxin-binding globulin (TBG) concentration and peripheral deiodination of thyroxine may be important pathways for estrogen to affect thyroid function [5]. Phytoestrogens are non-steroidal polyphenolic compounds commonly found in soy products and supplements [6], whose molecular structure is similar to that of the human estrogen 17-estradiol, so they have partial oestrogen activity. Studies have shown that phytoestrogens interfere with thyroxine synthesis and iodide absorption [7, 8]. In addition, high isoflavone intake may increase the risk of disease progression in patients with SCH [9]. However, the relationship between phytoestrogens and thyroid hormone sensitivity is still rarely reported. The aim of this study was to investigate the association between urinary phytoestrogens and thyroid function and its effect on thyroid hormone sensitivity in a large sample population.

Study Population

The National Health and Nutrition Examination Survey (NHANES) is a program of the National Center for Health Statistics (NCHS) in United States designed to assess the health and nutritional status of adults and children. The survey examines a nationally representative sample of about 5,000 persons each year. A total of 20,686 participants took part in the NHANES survey in 2007–2010. Of these, a total of 5,659 participants underwent urinary phytoestrogen concentration testing and 9,461 participants underwent thyroid function testing.

In this study, a total of 8,584 participants who had completed thyroid function tests and had complete thyroid function markers were initially selected. Then, 695 participants with self-reported thyroid disease and taking thyroid drugs (liothyronine, levothyroxine, thyroid desiccated, thyroid hormones - unspecified, propylthiouracil, or methimazole) were excluded. After assessing thyroid function, we excluded 426 participants with abnormal thyroid function (including subclinical thyroid dysfunction). In addition, we excluded other 2,210 participants lacking demographic data and data on other important covariates. After excluding 2,731 individuals with missing data on urinary phytoestrogens, 2,521 individuals were finally included in the final analysis. The detailed process of inclusion and exclusion is shown in Fig. 1.

Thyroid Function and Urinary Phytoestrogens

Thyroid blood specimens were processed, stored and shipped to the lab of Washington University (Seattle, WA) for testing. A two-step enzyme immunoassay was used for free thyroxine (FT4) testing, a competitive binding immunoenzymatic assay was used for total thyroxine (TT4), free triiodothyronine (FT3) and total triiodothyronine (TT3). The access hypersensitive human thyroid-stimulating hormone (TSH) assay is a 3rd generation, two-site immunoenzymatic (“sandwich”) assay. Normal thyroid function is defined by TT4 and TSH levels, with a TT4 of 4.5–13.2 ug/dl and a TSH of 0.39–4.5 uIU/L considered euthyroid.

In the 2007–2010 NHANES survey, urinary concentrations of 6 phytoestrogens were measured in part participants, including genistein, daidzein, equol, O-desmethylangolensin, enterodiol, and enterolactone. High performance liquid chromatography-atmospheric pressure photoionization-tandem mass spectrometry (HPLC-APPI-MS/MS) was used for the quantitative detection. Urine samples were processed, stored at -20°C and transported to the lab of the National Centre of Environmental Health for testing.

The above staff, including those who collect and test blood and urine samples, are professionally trained to ensure the required skill level. And, a variety of methods were used for NHANES Working Group to monitor the quality of the analyses performed by the contract laboratories to ensure the accuracy and precision of the tests concerned. Further details on laboratory testing and quality control can be found on the websites (www.cdc.gov/nchs/nhanes/index.htm; viewed on July 25, 2024).

Other Covariaties

Demographic variables including age, sex, race/ethnicity, education level and family income were included in our study. All participants were categorised by race as non-Hispanic white, non-Hispanic black, Hispanic and other. Education level was classified as < 8 years for 'less than high school', 8–12 years for 'high school or equivalent', > 12 years for 'more than high school'. Family income was stratified by quartiles (Q1 <$20000, Q2 $20000-$34999, Q3 $35000-$74999, Q4 ≥$75000).

Meanwhile, we included four lifestyle-related dichotomous variables in the analysis: tobacco intake, alcohol intake, vigorous work activity and vigorous recreational activities. Tobacco and alcohol consumption were cut off at 'at least 100 cigarettes in lifetime' and 'at least 12 drinks of alcohol in lifetime'. Vigorous work activity and vigorous recreational activities were defined by whether the work or recreational activities resulted in increased breathing or heart rate. Laclaustra M et al [10] showed that impaired thyroid hormone sensitivity is associated with diabetes and metabolic syndrome. Therefore, we included body mass index (BMI, kg/m²) and diabetes mellitus (DM) together as covariates. In this study, all participants were diagnosed with DM based on self-reported history of DM, fasting blood glucose, and fasting glycated haemoglobin.

Indices of thyroid hormone sensitivity

Thyroxine is present in peripheral blood in the form of FT3 and FT4, with FT3 being less abundant but more active. FT4 can be converted to FT3 under the action of deiodinase. Therefore, the FT3/FT4 ratio represents the ability to convert FT4 to FT3, reflecting deiodinase activity and peripheral thyroid hormone sensitivity, with higher ratios implying higher peripheral blood thyroid hormone activity [11, 12].

Central sensitivity to thyroid hormone is a reflection of the pituitary gland's ability to respond to thyroxine. Thyroid Feedback Quantile-based Index (TFQI), which stands for thyroid feedback quantile-based index, is a new Resistance to Thyroid Hormone Index [10]. TFQI was calculated as cdf (FT4) - (1 - cdf (TSH)). cdf is the cumulative distribution function of a continuous numerical variable. This index ranges between − 1 and 1, and larger values indicate a reduced central sensitivity to FT4. TFQI is more stable than TT4RI and TSHI at abnormal ranges of the thyroid gland responsiveness to TSH due to the narrow range of values observed. The results do not present any extreme values, even in individuals with significant thyroid dysfunction.

Statistical Analysis

Due to the skewed distribution of urinary phytoestrogen concentrations, we converted the urinary phytoestrogen concentration data to normal distribution by taking the logarithm. For baseline analysis, data that was normally or approximately normally distributed was reported as mean ± standard deviation (SD). Skewed data was reported as median (interquartile spacing), while categorical variables were reported as the number (percentage). When comparing inter-groups differences, normal continuous variables were analysed using the chi-squared test or ANOVA, and non-normal continuous and categorical variables were analysed using the KruskalWallis test.

A linear regression model were used to analyse the association of different urinary phytoestrogens with thyroid function and thyroid hormone sensitivity. All analyses were adjusted for age, sex, race/ethnicity, education level, family income, BMI, tobacco intake, alcohol intake, vigorous work activity, vigorous recreational activities, and DM. As oestrogen levels vary significantly between the sexes, we performed a stratified analysis by sex. In view of the fact that thyroid function varies greatly with aging, we further stratified by age.

All analyses in this study were performed using R version 4.3.2 (R Foundation for Statistical Computing, Vienna, Austria) and a two-tailed P-value < 0.05 was considered statistically significant.

Baseline Characteristics

After screening with strict inclusion and exclusion criteria, a total of 2,521 participants aged ≥ 20 years were included in the final analysis, the baseline characteristics are shown in Table 1. The median age of the population was 48 years, of which 1,336 were male (52.99%) and 1,185 were female (47.01%).

Table 1

Basic characteristics of study population.
	^a Range (mean ± SD, median (P25, P75)) or N (percentage)
Variables	Over all	Female (N = 1185)	Male (N = 1336)	P-value
Age	48 (35, 62)	46 (33, 61)	50 (36, 64)	0.001*
Race
Non-Hispanic white	1170 (46.4%)	520 (43.9%)	650 (48.7%)	0.095
Non-Hispanic black	481 (19.1%)	231 (19.5%)	250 (18.7%)
Mexican-American	502 (19.9%)	252 (21.3%)	250 (18.7%)
Other races	368 (14.6%)	182 (15.4%)	186 (13.9%)
Education level
≤8 years	723 (28.7%)	343 (28.9%)	380 (28.4%)	0.438
9–12 years	1314 (52.1%)	627 (52.9%)	687 (51.4%)
>12 years	484 (19.2%)	215 (18.1%)	269 (20.1%)
Family income (dollar)
Q1 (< 20000)	598 (23.7%)	305 (25.7%)	293 (21.9%)	0.084
Q2 (20000–34999)	613 (24.3%)	292 (24.6%)	321 (24.0%)
Q3 (35000–74999)	733 (29.1%)	324 (27.3%)	409 (30.6%)
Q4 (≥ 75000)	577 (22.9%)	264 (22.3%)	313 (23.4%)
BMI (Kg/m²)
<19.0	50 (2.0%)	30 (2.5%)	20 (1.5%)	0.429
19.0-24.9	665 (26.4%)	342 (28.9%)	323 (24.2%)
25.0-29.9	844 (33.5%)	354 (29.9%)	490 (36.7%)
≥30	962 (38.2%)	459 (38.7%)	503 (37.6%)
Tobacco intake
No	1304 (51.7%)	729 (61.5%)	575 (43.0%)	< 0.001*
Yes	1217 (48.3%)	456 (38.5%)	761 (57.0%)
Alcohol intake
No	645 (25.6%)	436 (36.8%)	209 (15.6%)	< 0.001*
Yes	1876 (74.4%)	749 (63.2%)	1127 (84.4%)
^b Vigorous work activity
No	1988 (78.9%)	1041 (87.8%)	947 (70.9%)	< 0.001*
Yes	533 (21.1%)	144 (12.2%)	389 (29.1%)
^b Vigorous recreational activities
No	1997 (79.2%)	1008 (85.1%)	989 (74.0%)	< 0.001*
Yes	524 (20.8%)	177 (14.9%)	347 (26.0%)
Diabetes Mellitus
No	2117 (84.0%)	1014 (85.6%)	1103 (82.6%)	0.04*
Yes	404 (16.0%)	171 (14.4%)	233 (17.4%)
Thyroid function indicators
FT3	3.17 (2.90, 3.40)	3.10 (2.00, 4.50)	3.25 (2.14, 5.20)	< 0.001*
FT4	10.30 (9.00, 11.20)	10.30 (5.20, 18.10)	10.30 (5.20, 19.40)	0.131
TT3	1.90 (2.60, 2.00)	1.90 (0.90, 3.60)	1.90 (0.90, 3.20)	0.667
TT4	7.70 (6.80, 8.70)	7.90 (4.50, 13.20)	7.50 (4.50, 13.10)	< 0.001*
TSH	1.52 (1.04, 2.19)	1.51 (0.39, 4.46)	1.55 (0.39, 4.49)	0.696
Indices of thyroid hormones sensitivity
FT3/FT4	0.32 (0.28, 0.36)	0.31 (0.13, 2.80)	0.32 (0.12, 0.67)	< 0.001*
TFQI	0.03 (-0.23, 0.28)	0.02 (-0.95, 0.93)	0.04 (-0.94, 0.90)	0.174
Urinary phytoestrogens
Daidzein (ng/ml)	4.10 ± 2.04	4.05 ± 2.12	4.15 ± 1.96	0.115
O-DMA (ng/ml)	1.37 ± 2.70	1.44 ± 2.82	1.31 ± 2.59	0.553
Equol (ng/ml)	1.82 ± 1.57	1.78 ± 1.59	1.85 ± 1.54	0.053
Enterodiol (ng/ml)	3.54 ± 1.89	3.57 ± 1.93	3.51 ± 1.86	0.268
Enterolactone (ng/ml)	5.40 ± 2.04	5.35 ± 2.07	5.45 ± 2.01	0.214
Genistein (ng/ml)	3.38 ± 1.94	3.28 ± 1.99	3.47 ± 1.88	< 0.01*
*P < 0.05
^a Normally distributed continuous variables are presented as mean ± standard deviation (SD) and non-normally distributed continuous variables are presented as median (interquartile). ^b Participants were asked if they had any vigorous work or recreational activities that would cause a large increase in breathing or heart rate. Abbreviations: BMI: Body mass index. FT3: Free triiodothyronine. FT4: Free thyroxine. TT3: Total triiodothyronine. TT4: Total thyroxine. TSH: Thyroid stimulating hormone. TFQI: Thyroid feedback quantile index. O-DMA: O-Desmethylangolensin.

The proportion of non-Hispanic whites was 46.4%, and 52.1% of participants had a high school education or equivalent. There were no significant differences in race, education level, family income, and BMI between participants of different sexes (P ≥ 0.05). The proportion of male participants who had smoked and drunk alcohol was 57.0% and 84.4% respectively, much higher than that of female participants (38.5% and 63.2%). Similarly, male participants were much more likely to have vigorous work activity (29.1%) and vigorous recreational activities (26.0%) than female participants (12.2% and 14.9%). A total of 404 participants had DM, 233 were men and 171 were women.

In terms of thyroid function, FT3 levels were higher in men (3.25 pg/ml) than in women (3.10 pg/ml), while TT4 levels were lower in men (7.50 ug/dl) than in women (7.90 ug/dl). There were no significant differences in other thyroid function indexes between different sexes. In terms of thyroid hormone sensitivity indices, the FT3/FT4 ratio was higher in men (0.32) than in women (0.31), but there was no statistical difference in the TFQI between the sexes. Of the 6 urinary phytoestrogens, only Genistein differed between the sexes. The mean concentration of Genistein in men was 3.47 ± 1.88 ng/ml, which was significantly higher than in women (3.28 ± 1.99 ng/ml).

Association between urinary phytoestrogens and thyroid function indicators

The concentration of urinary phytoestrogens was associated with FT4 and TT4 level in the overall analysis (Table 2). Of the 6 phytoestrogens, one was associated with FT4 and three types with TT4. Enterodiol was negative associated with FT4 (P = 0.03), Equol, Enterodiol and Enterolactone were negatively associated with TT4 (P < 0.01). As urinary phytoestrogens concentration increased, FT4 and TT4 showed a decreasing trend.

Table 2

The associations between urinary phytoestrogens and thyroid function.
Urinary Phytoestrogens	FT3		FT4		TT3		TT4		TSH
Urinary Phytoestrogens	Std. β	P-value	Std. β	P-value	Std. β	P-value	Std. β	P-value	Std. β	P-value
Over all
Daidzein (ng/ml)	0.006	0.71	0.026	0.19	0.016	0.41	-0.005	0.81	0.015	0.43
O-DMA (ng/ml)	0.001	0.95	0.038	0.06	0.011	0.57	-0.019	0.33	0.018	0.36
Equol (ng/ml)	0.001	0.96	-0.027	0.18	-0.029	0.14	-0.055	< 0.01*	0.013	0.50
Enterodiol (ng/ml)	-0.002	0.90	-0.043	0.03*	0.031	0.11	-0.055	< 0.01*	0.032	0.10
Enterolactone (ng/ml)	-0.013	0.45	0.014	0.48	-0.009	0.66	-0.058	< 0.01*	0.023	0.24
Genistein (ng/ml)	0.012	0.49	0.008	0.70	0.029	0.13	< 0.001	0.98	0.011	0.58
Male participants
Daidzein (ng/ml)	0.014	0.55	-0.028	0.32	0.011	0.69	0.010	0.71	0.016	0.56
O-DMA (ng/ml)	0.004	0.85	-0.010	0.72	0.029	0.28	-0.006	0.83	0.022	0.42
Equol (ng/ml)	0.011	0.66	-0.046	0.10	-0.013	0.62	-0.062	0.03*	0.003	0.91
Enterodiol (ng/ml)	0.019	0.41	-0.023	0.40	0.035	0.18	-0.084	< 0.01*	-0.020	0.47
Enterolactone (ng/ml)	0.014	0.55	0.020	0.47	0.010	0.71	-0.044	0.11	0.048	0.08
Genistein (ng/ml)	0.022	0.35	-0.042	0.13	0.027	0.30	0.006	0.83	0.011	0.67
Female participants
Daidzein (ng/ml)	0.006	0.82	0.088	< 0.01*	0.027	0.34	-0.024	0.41	0.014	0.63
O-DMA (ng/ml)	0.006	0.83	0.093	< 0.01*	0.003	0.91	-0.033	0.25	0.013	0.65
Equol (ng/ml)	-0.006	0.84	-0.006	0.83	-0.038	0.18	-0.047	0.10	0.024	0.42
Enterodiol (ng/ml)	-0.015	0.60	-0.067	0.02*	0.028	0.32	-0.035	0.22	0.088	< 0.01*
Enterolactone (ng/ml)	-0.031	0.26	0.010	0.73	-0.024	0.41	-0.035	< 0.01*	-0.002	0.95
Genistein (ng/ml)	0.007	0.80	0.068	0.02*	0.036	0.21	-0.006	0.83	0.009	0.75
Std. β: Standardized β-value.

After sex stratification, the association between the urinary phytoestrogens and thyroid function showed a significant difference. In male participants, the association between phytoestrogens and FT4 disappeared, while the negative association between Equol, Enterodiol and TT4 remained stable. In female participants, four types phytoestrogens were associated with FT4 levels. It is worth noting that the levels of Daidzein, O-Desmethylangolensin (O-DMA) and Genistein were positively associated with FT4, whereas Enterodiol were negatively associated with FT4. Of the three phytoestrogens associated with TT4, only Enterolactone remained stable. In addition, Enterodiol was found to be positively associated with TSH levels.

Association between urinary phytoestrogens and thyroid hormone sensitivity indices

In the overall analysis, there was no significant association between urinary phytoestrogens and the FT3/FT4 ratio (Fig. 2), while there were significant associations between urinary phytoestrogens and the TFQI. TFQI increased with the increasing concentration of urinary O-DMA and Enterolactone. There was no association between the other 4 types of urinary phytoestrogens.

After sex stratification, the association between urinary phytoestrogens and thyroid hormone sensitivity indices also showed significant sex differences (Table 3). Urinary phytoestrogens turned to be associated with FT3/FT4 ratio in both men and women. In men, Genistein was positively associated with the FT3/FT4 ratio (P = 0.04), whereas in women, Daidzein and O-DMA were negatively associated with the FT3/FT4 ratio (P < 0.05). In terms of the association between urinary phytoestrogens and TFQI, the association of Enterolactone remained stable in men. O-DMA and Daidzein showed a positive association with TFQI in women.

Table 3

The associations between urinary phytoestrogens and thyroid hormone sensitivity indices after sex stratification.
Variables	FT3/FT4			TFQI
Variables	Std. β	Std. Error	P-value	Std. β	Std. Error	P-value
Male participants
Daidzein (ng/ml)	0.046	0.026	0.08	-0.007	0.027	0.79
O-DMA (ng/ml)	0.024	0.026	0.37	0.015	0.027	0.57
Equol (ng/ml)	0.046	0.027	0.08	-0.035	0.027	0.20
Enterodiol (ng/ml)	0.035	0.026	0.19	-0.030	0.027	0.26
Enterolactone (ng/ml)	-0.011	0.027	0.69	0.060	0.028	0.03*
Genistein (ng/ml)	0.055	0.026	0.04*	-0.025	0.027	0.35
Female participants
Daidzein (ng/ml)	-0.070	0.028	0.01*	0.058	0.029	0.04*
O-DMA (ng/ml)	-0.078	0.028	< 0.01*	0.072	0.029	0.01*
Equol (ng/ml)	>-0.001	0.029	0.99	0.016	0.029	0.59
Enterodiol (ng/ml)	0.039	0.028	0.17	0.010	0.029	0.72
Enterolactone (ng/ml)	-0.030	0.029	0.29	0.026	0.029	0.37
Genistein (ng/ml)	-0.048	0.028	0.09	0.041	0.029	0.15

The association between phytoestrogens and thyroid function is still unclear, and relevant studies have not found that soy foods or isoflavones have adverse effects on thyroid function in euthyroid people [9]. However, most of the above studies had limited sample sizes and the credibility of their results is questionable. In our study, a large sample cross-sectional analysis using the NHANES database showed that urinary phytoestrogens were significantly negatively associated with FT4 and TT4 levels, even in people with normal thyroid function.

Higher urinary phytoestrogens were associated with lower TT4 levels in both men and women. TT4 is the sum of serum free and bound thyroxine, which is the most basic index of thyroid synthesis ability. Thyroid hormone synthesis depends on the activity of thyroid peroxidase (TPO). Studies have shown that plant flavonoids affect thyroxine synthesis by inhibiting TPO through various mechanisms, and isoflavones can inhibit the uptake of iodide by thyroxine and also affect thyroxine synthesis [7]. This may be one of the reasons why urinary phytoestrogens were negatively associated with TT4 levels.

More than 99.7% of thyroid hormone in the peripheral circulation is in conjugated form, the vast majority of which binds to thyroxin-binding globulin (TBG). Estrogens can promote TBG synthesis in the liver, thereby affecting plasma thyroxine levels [13]. Normally, FT4 should show a downward trend as thyroid hormone synthesis decreases and TBG synthesis increases. In female participants, we found that Enterodiol was negatively associated with FT4 level and positively associated with TSH level, which is consistent with the above condition.

It is notably that Daidzein, O-DMA and Genistein were positively associated with FT4 levels in female. In addition to TBG, FT4 levels were also affected by the activity of peripheral deiodinase, which is responsible for converting FT4 to FT3. Animal study had shown that after 12 months of consumption of soy phytoestrogens in dogs, serum T4 level increased but T3 level did not change, which may be related to inhibition of 50-deiodinase activity [14]. de Souza Dos Santos MC et al [15] found that phytoestrogens have a broad inhibitory effect on 1-deiodinase, and the inhibitory effect of different phytoestrogens varies.

FT3 is the main form of thyroid hormone that exerts biological effects and is also one of the most sensitive indicators of thyroid function. FT3 is closely related to the body's metabolism condition and various diseases [16,17]. In our study, there was no association between urinary phytoestrogens and FT3 levels, suggesting that urinary phytoestrogens may not be sufficient to influence the overall metabolic condition of the body. However, in order to maintain the FT3 level required for normal body metabolism, there must be a sufficient FT4 concentration in the presence of decreased deiodinase activity. This may be one of the reasons for the positive correlation between the above 3 phytoestrogens and FT4. There may be some feedback mechanism, but more evidence is needed to confirm this.

We then further analyzed the association between urinary phytoestrogens and thyroid hormone sensitivity indices. In the overall analysis, two types of urinary phytoestrogens were positively associated with TFQI. After sex stratification, there was a positive association between phytoestrogens and FT3/FT4 and TFQI in men. In women, urinary phytoestrogens were negatively associated with FT3/FT4 and positively associated with TFQI.

The FT3/FT4 ratio of the peripheral thyroid sensitivity index is a direct reflection of peripheral deiodinase activity. The negative association between Daidzein and O-DMA and FT3/FT4 ratio in women may be related to the extensive inhibition of phytoestrogens on deiodinase [15]. Selenocysteine residues in the active site of deiodinase contribute to the removal of iodine by halogen bonding with selenium, while compounds such as polychlorinated biphenyls, polybrominated diphenyl ethers and their hydroxylated metabolites interfere with the activity of deiodinase by binding to selenocysteine residues in the active site of deiodinase [18]. Whether phytoestrogens inhibit deiodinase by the same mechanism in humans still needs further study.

Testosterone can increase deiodinase activity, a study by Bisschop PH et al [19] of 50 transsexuals with normal thyroid function found that testosterone increased the FT3/FT4 ratio. In addition, the inhibitory effect of phytoestrogens on aromatase resulted in a decrease in estrogen production and an increase in androgen levels. The above reason may partly explain the positive association between Genistein and FT3/FT4 ratio in male participants.

Meanwhile, our study also indicated that urinary phytoestrogens were positively associated with TFQI in both men and women, suggesting that some types of phytoestrogens may adversely affect the central sensitivity of thyroxine. But the exact mechanism remains unknown. As known that any alterations in thyroid hormone synthesis, transport and metabolism that result in a reduced response of peripheral tissues to thyroid hormone can cause RSTH. The most common and dominant of these are thyroid hormone receptor targets.

Studies have confirmed that mutations in both the α-receptor and β-receptor of thyroxine can cause impaired thyroid hormone sensitivity, resulting in elevated thyroxine and TSH concentrations [20,21,22]. In addition, estrogen receptors and thyroid hormone receptors both are members of the nuclear receptor superfamily, and studies have confirmed that there can be extensive interactions between members of the nuclear receptor superfamily [23]. This may be an evidence that phytoestrogens influence thyroid hormone sensitivity through thyroid hormone receptors.

In summary, the associations between urinary phytoestrogens and thyroid function and thyroid hormone sensitivity is complex (Figure 3). Which suggest that phytoestrogens may affect the synthesis, transport and metabolism of thyroid hormones at several levels. However, the site of action of different phytoestrogens and the intensity of their effect can be quite different. Among the 6 types of urinary phytoestrogens detected in NHANES study, Daidzein and Genistein are Soy Isoflavones, O-DMA and Equol are secondary metabolites of Daidzein. While Enterodiol and Enterolactone are metabolites of lignans under the action of intestinal flora. This may be one of the reasons why different phytoestrogens have different effects.

In addition, phytoestrogens undergo further metabolism in the gut and body to cause structural changes such as glycosylation, hydroxylation and methylation, resulting in a variety of secondary metabolites. Various structural changes cause different phytoestrogens to have different effects on estrogen receptors and thyroid hormone receptors. Furthermore, different expression of different estrogen receptors (ERs) in different tissues, resulting in completely different physiological effects [24]. The above reasons can also explain the diversity of association between different urinary phytoestrogens and thyroid function.

Epidemiological studies show that the incidence of SCH is increasing in the elderly population and that TSH increases with age [25]. In our study, thyroid hormone sensitivity showed a significant downward trend with the increasing age (Figure 4). As RSTH is one of the main causes of hypothyroidism, there is reason to believe that RSTH may increase the risk of SCH and even hypothyroidism in the elderly population. Therefore, it is necessary to monitor the health of people who consume excessive amounts of phytoestrogens, especially Asian people who prefer to eat soy products. Statistically, the average consumption of isoflavones in Western countries is known to be 2 mg/day, while the average consumption in some Asian countries can be as high as 15-50mg/day [26,27]. Our study was based on European populations, and the association may be more significant in Asian populations, but there is still a lack of data on this.

Limitations

Our study is a cross-sectional analysis, and the lack of longitudinal follow-up data is not sufficient to directly confirm the causal relationship between urinary phytoestrogens and decreased thyroid hormone sensitivity. Further studies are needed to confirm the association. In addition, due to the limited amount of data, we did not further stratified by age and could not confirm the association between urinary phytoestrogen concentrations and thyroid function and thyroid sensitivity indicators at all ages.

Urinary phytoestrogens were closely related to thyroid function and thyroxine sensitivity indices, and there were significant sex differences in the association. Increased urinary phytoestrogens are associated with decreased thyroxine sensitivity, suggesting a possible increased risk of SCH and hypothyroidism, especially in older adults with decreased thyroxine sensitivity.

Author Statements

All authors declare that this study does not have any potential or actual conflicts of interest in business or financial relationships.

Author Contribution

Zongbao Li was responsible for study design, data analysis and writing of the original draft. Fan Hu was responsible for study design, results interpretation and critical editing. Han Yu provided support in data sorting and article editing. Yanhui Lu was responsible for study design, results interpretation, critical editing and article approval.

Acknowledgments

The authors are grateful for the dedication of all study participants and the scientific rigour of the NHANES Working Group, which provided the important data support for this study.

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No competing interests reported.

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Urinary phytoestrogens were associated with reduced sensitivity to thyroid hormone in NHANES 2007-1010: A large cross-sectional study

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Abstract

Background

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BACKGROUND

METHODS

Study Population

Thyroid Function and Urinary Phytoestrogens

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Indices of thyroid hormone sensitivity

Statistical Analysis

RESULT

Baseline Characteristics

Association between urinary phytoestrogens and thyroid function indicators

Association between urinary phytoestrogens and thyroid hormone sensitivity indices

DISCUSSION

CONCLUSION

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Author Statements

Author Contribution

Acknowledgments

References

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