Subclinical Hypothyroidism and Incident Depression in Young and Middle-Age Adults.

Background: The role of subclinical hypothyroidism in the development of depression remains controversial. We examined the prospective association between subclinical hypothyroidism and incident depressive symptoms.

Methods: We conducted a prospective cohort study of 220,545 middle-age adults without depression who had undergone at least two comprehensive health examinations between 1 January 2011 and 31 December 2014. Thyroid-stimulating hormone, free triiodothyronine (FT3), and free thyroxine (FT4) levels were measured using an electrochemiluminescent immunoassay. The study outcome was incident depressive symptoms, defined as a Center for Epidemiologic Studies-Depression score >16.

Results: During a median follow-up period of 2 years, incident depressive symptoms occurred in 7323 participants. The multivariable-adjusted hazard ratio for incident depressive symptoms comparing subclinical hypothyroid and euthyroid participants was 0.97 (95% confidence interval, 0.87 to 1.09). Similarly, among euthyroid participants (n = 87,822), no apparent association was found between thyroid hormone levels and an increased risk of incident depressive symptoms.

Conclusions: No apparent association was found between subclinical hypothyroidism and incident depressive symptoms in a large prospective cohort of middle-age men and women.