In depressed individuals, there is an increased incidence of subclinical hypothyroidism.  While there have been few studies on the role of subclinical hypothyroidism in the development of depression, this topic remains controversial. Few cohort studies exist examining the association between these disease states; however, a majority of these studies focused on elderly populations. The Journal of Clinical Endocrinology and Metabolism recently published a prospective study evaluating the association between subclinical hypothyroidism and incident depression in young and middle-aged adults.

This prospective cohort study included 220,545 middle-aged Korean adults with a median follow-up period of 2 years. Serum-free triiodothyronine (T3), free thyroxine (T4), and thyroid-stimulating hormone (TSH) levels were measured in order to either diagnose participants with subclinical hypothyroidism or classify them as euthyroid. The primary endpoint was the development of incident depressive symptoms in this study population.

Incident depression was defined as a score of greater than 16 according to the Center for Epidemiologic Studies Depression Scale. Investigators compared the risk of incident depressive symptoms between the two groups. In euthyroid participants, the dose-response relationship between thyroid hormone levels and depressive symptoms was determined.

Study results revealed that incident depressive symptoms occurred in 7,323 participants. The hazard ratio for incident depressive symptoms in subclinical hypothyroid versus euthyroid participants was 0.95 [95%, 0.85-1.09]. Additionally, there was no link between thyroid hormone levels and an increased risk of incident depression in euthyroid participants.

Investigators concluded there was no association between subclinical hypothyroidism and incident depressive symptoms in young and middle-aged Korean adults. This lack of association was similar throughout various age and sex groups and was not affected when adjusting for potential confounders. Though previously published studies reported positive associations between subclinical hypothyroidism and depression, this study is the largest prospective cohort study to evaluate this association.

Findings established no apparent association between subclinical hypothyroidism and incident depressive symptoms; however, these results may not be generalizable to other ethnicities and age groups as study participants were middle-aged Korean men and women. Further studies among various patient populations are needed to confirm the findings of this study. While this topic remains controversial, this study provides a better understanding of incident depression among patients with subclinical hypothyroidism.

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