This study investigated the prevalence and associated factors of abnormal thyroid function in a large sample of first-time inpatients with dyslipidemic MDD. The main findings were as follows: (1) The prevalence of thyroid dysfunction among dyslipidemic MDD patients was 39.97%. (2) Risk factors for thyroid dysfunction in dyslipidemic MDD patients included disease duration, age at onset, HAMA score, and LDL-C levels. (3) Course of disease, age at onset, HAMA score, HAMD score, FBG, and SBP levels significantly influenced serum TSH levels.
Our study showed that the prevalence of dyslipidemic MDD patients with thyroid dysfunction was 39.97%. There are many reports on the prevalence of thyroid dysfunction diagnosed by MDD, but the reported prevalence is highly heterogeneous, and several factors are potentially responsible for this heterogeneity. In a previous study of first-episode and drug-naïve MDD patients in a Chinese population, the prevalence of subclinical hypothyroidism was 60.7% [24], which is much higher compared to the prevalence of thyroid dysfunction in our current study (39.97%), But the study did not differentiate between lipid grades (Lipid metabolism is closely related to thyroid function[25, 26]). In contrast, in a study on the association of depression with comorbid thyroid disease in a European population, the point prevalence of MDD combined with hypothyroidism was found to be 13.2%[27], which is lower than the prevalence found in our study, which may be related to the ethnic background of the subjects. It has been shown that the development of hypothyroidism is closely related to genetic factors, firstly, the TSH receptor is currently the only receptor that can transduce TSH signals, and inactivation of the TSH receptor gene leads to tolerance of the body to TSH, which is one of the causes of hypothyroidism [28]. In addition, gender is also a factor influencing abnormal thyroid function in patients with MDD, Gorkhali et al. reported that thyroid dysfunction was more common in female patients with MDD[13]. In conclusion, in our study, dyslipidemic MDD patients had a high prevalence of thyroid dysfunction.
Another important result of this study was the finding that the course of disease, onset age, HAMA scores, and LDL-C levels were risk factors for comorbid thyroid dysfunction in patients with dyslipidemic MDD. A study on the effect of thyroid function on the risk of readmission after hospital discharge in patients with MDD showed that the longer the duration of disease in MDD patients, the more likely they were to develop thyroid dysfunction, and the worse the patient's outcome and prognosis [29]. One study showed that patients with MDD at a later age of onset had higher TSH levels, and anxious MDD patients had higher TSH levels than those without anxiety [30], which suggests that a later age of onset and a high HAMA score are more likely to be associated with thyroid dysfunction. Thyroid stimulating hormones play an important role in maintaining thyroid function, of which lipid metabolism is a key regulator, and hypothyroidism is a common endocrine disorder associated with dyslipidemia [31]. A study conducted in an Indian population on response to levothyroxine treatment in patients with subclinical hypothyroidism showed higher LDL-C levels in the subclinical hypothyroidism group compared to the control group [32], which is consistent with our study.
We also found that onset age, LDL-C levels, HAMA score, HAMD score, FBG, and SBP significantly affects serum TSH levels. We found that HAMA scores significantly affect serum TSH levels, which is in agreement with several other studies [24, 33–35]. The results of these studies suggest that the severity of depressive symptoms and the severity of anxiety symptoms in MDD significantly affect serum TSH levels. This is probably due to the fact that TSH is involved in mood regulation by acting on the thyroid hormone receptor α and β subtypes, both of which are expressed in the limbic system [36, 37]. TSH may also affect mood by modulating the brain’s serotonergic system and the γ-aminobutyric acidergic system [38, 39]. In terms of metabolism, we found that FBG and LDL-C significantly affect serum TSH levels, which is consistent with the view of Zhu et al. [40]. A study has shown that abnormal thyroid function can increase the risk of developing diabetes and diabetic complications[41]. Previous studies have suggested a possible common etiology of MDD, thyroid disease, and diabetes, namely an imbalance in the neuroendocrine network[42]. In addition, previous studies validated our view that LDL-C significantly affects TSH levels[43]. In terms of blood pressure, it has been demonstrated that thyroid dysfunction is associated with hypertension[44], and our study found that systolic blood pressure was independently and correlated with thyroid dysfunction in MDD patients with dyslipidemia, which may be due to thyroid dysfunction causing abnormalities in lipid metabolism and subsequently atherosclerosis, which ultimately leads to changes in blood pressure[45].
Several limitations need to be noted in this study. First, the cross-sectional design of this study did not allow for establishing any causal relationship between thyroid function and clinical variables in patients with MDD, and future studies should use longitudinal studies to reveal the causal relationship between these effects and thyroid dysfunction. Second, other possible influences such as smoking, alcohol consumption, and medication use were not collected in our study. These factors play an important role in the development of thyroid function, lipid metabolism, and MDD, and the inclusion of more influential factors in future studies would help to better understand the mechanisms of disease development. Third, although these patients are currently diagnosed with MDD, the possibility that some of these diagnoses may change to bipolar disorder or other psychiatric disorders in the future cannot be ruled out.
In conclusion, this study found a high prevalence of thyroid dysfunction in patients with MDD dyslipidemia. Age of onset, TC level, HAMA score, HAMD score, FBG level, and SBP level significantly influenced serum TSH level, which provides a direction for the treatment of thyroid dysfunction in dyslipidemic patients with MDD.