Association of Thyroid Function With Depression: A Historical Cohort Study.

Background: There is inconsistent evidence concerning the association between thyroid dysfunctions and depression. Therefore, we conducted this population-based study to investigate the relationship between thyroid-stimulating hormone (TSH) and depression using the Patient Health Questionnaire-9 (PHQ-9).

Method: We included patients (≥18 y) who received care at the Marshfield Clinic Health System and completed a TSH level and a PHQ-9 within 2 weeks, between 2015 and 2020. We used a logistic regression model adjusted for covariates to estimate the effect of TSH level on clinically relevant depression (PHQ-9 score ≥10).

Results: Our cohort included 31,099 patients, mean age 50.9±19.5 years, 73.5% females, and 96.3% White, 39.7% of whom had clinically relevant depression. The patients with clinically relevant depression had higher TSH levels compared with those without such depression (P=0.01). In this cohort, we found that low TSH levels (<0.5 mIU/L) and very elevated TSH levels (≥10 mIU/L) both had increased odds of being associated with clinically relevant depression: [1.30 (95% CI: 1.15-1.46) and OR 1.50 (95% CI: 1.25-1.81), respectively]. Subgroup analysis showed similar findings in patients with mood disorders, females, younger adults (<65 y), and those with untreated thyroid disorders. In males, only low TSH was associated with clinically relevant depression, whereas in patients with treated thyroid disorders, only very elevated TSH was associated with clinically relevant depression.

Conclusions: In this large cross-sectional study, we found that low TSH and very elevated TSH levels were both associated with higher odds of depression. Similar trends were observed in patients with mood disorders, females, younger adults (<65 y), and those with untreated thyroid disorders. Large prospective population-based studies are needed to further investigate the relationship between TSH levels and clinical depression.