The Association Between Thyrotropin and Clinically Relevant Depression: A Retrospective Cross-Sectional Study

<b><i>Introduction:</i></b> Thyroid dysfunction and mood disorders are chronic health conditions with a significant impact on quality of life. This study aimed to investigate the association between thyrotropin (TSH) and clinically relevant depression (CRD) in patients with and without mood disorders, in a population-based sample. <b><i>Methods:</i></b> This retrospective cross-sectional study included all consecutive adults (≥18 years) who had TSH and completed the Patient Health Questionnaire (PHQ-9) within 6 months of the index visit, between October 2016 and May 2021, at the University of Utah Health. Data on demographics, hypothyroidism diagnoses, TSH, thyroid hormone replacement (THR), PHQ-9, antidepressant (AD) medications, and mood disorder diagnoses (using the International Classification of Diseases, 10th Revision, Clinical Modification codes; Major depressive disorder single episode-F32, recurrent-F33, persistent mood disorder-F34, bipolar disorder-F30+F31, and mood disorder not otherwise specified-F39) were extracted electronically. CRD was defined as PHQ-9 ≥ 10. <i>t</i>-Test and chi-square test were used to compare continuous and categorical variables, respectively. Logistic regression models were formulated to evaluate the association between TSH and CRD, after adjusting for covariates. <b><i>Results:</i></b> The cohort included 33,138 patients, mean age 42.41 ± 16.10 years, 80.67% Caucasian, 69.10% females, and mean PHQ-9 score 10.11 ± 6.94. A total of 45.23% (<i>n</i> = 14,989) patients had a diagnosis of mood disorders, and 49.70% had CRD. Patients with mood disorders were more likely to be female, Caucasian, non-Hispanic/Latino, on AD, had hypothyroidism diagnoses, on thyroid medications, had higher mean PHQ-9 scores, and had CRD. TSH level was associated with an increased odds of CRD (odds ratio [OR] = 1.01, confidence interval [CI], 1.01-1.02, <i>p</i> < 0.009) after adjusting for age, sex, body mass index, Charlson Comorbidity Index, and use of THR and AD. Both the low TSH and high TSH groups showed increased odds of CRD, with respective ORs of 1.19 (CI: 1.04-1.37) and 1.26 (CI: 1.13-1.40). <b><i>Conclusions:</i></b> Thyroid dysfunction is associated with an increase in the odds of depression. Future longitudinal cohort studies are recommended to investigate the association between thyroid function and incident depression.