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Question: What is the relationship between thyroid dysfunction and mood disorders? 

Alan F. Schatzberg, M.D.: The relationship between activity of the hypothalmus, pituitary gland and thyroid gland (HPT axis) and depression has been studied for many years. Hypothyroidism – a syndrome of low output of the thyroid gland – is associated with symptoms of fatigue and depression.

There are two forms of clinical hypothyroidism (Types I and II) differentiated primarily by how underactive the thyroid gland is. The vast majority of patients who have symptoms of major depression do not have Type I or Type II hypothyroidism. There is however, another type of subtle early-stage hypothyroidism. In this syndrome, thyroid output is essentially normal, but antithyroid antibodies are present, suggesting a problem with the immune system and its reaction to thyroid tissue. Although this is relatively common (20-30%) in depressed patients, its significance is not yet clear.

As a rule, patients with symptoms of depression should have routine physical and laboratory examinations, including thyroid screening tests which check thyroid stimulating hormone (TSH) levels. TSH levels indicate how much the pituitary gland is activating the thyroid gland. Thyroid antibody levels should be checked if TSH levels are abnormal or possibly if depression has been lengthy.

Triiodothyronine (T3) is a hormone used to compensate for an underactive thyroid that is commonly used in patients who have not responded to antidepressants. T3 is derived from thyroxine (T4). T4 appears to have less of an effect on depression, so some investigators have argued that depressed patients may have an abnormality in converting T4 to T3.

Transthyretin – a transport protein – may also be low in patients with treatment-resistant depression. Further studies on thyroid function in depression are necessary and ongoing.

Alan F. Schatzberg, M.D., is the Kenneth T. Norris, Jr. Professor in Psychiatry & Behavioral Sciences and Chairman of the Department of Psychiatry and Behavioral Sciences at Stanford University School of Medicine. He is also a member of DBSA’s Scientific Advisory Board.