Subclinical thyroid dysfunction and major depressive disorder.

Purpose: This review attempts to investigate the link between subclinical thyroid dysfunction (SCH) and major depressive disorder (MDD). It has been speculated that SCH may be related to MDD through an autoimmune mechanism.

Methods: A comprehensive literature search was conducted in the PubMed database for relevant research and review articles.

Results: There appears to be an association between an autoimmune mechanism, possibly involving the thyroid gland, and depressive disorders, but the available evidence is so far inconclusive.

Conclusion: Lifetime prevalence of depression is significantly higher in patients with SCH, a finding reflecting a possible effect of SCH in lowering the threshold for the emergence of MDD. The relationship between SCH and MDD is, however, not clear, with large and well-designed studies investigating possible links between reference-range thyroid hormone levels and MDD having as yet found no relation between the two.