Sometimes depression results from an underactive thyroid, from the Harvard Mental Health Letter
Nearly 10 million Americans suffer from hypothyroidism, a condition in which the thyroid gland is underactive. In some people, hypothyroidism may be mistaken for depression. These individuals may be prescribed an antidepressant, rather than the thyroid medication that they really need.
The July 2011 issue of the Harvard Mental Health Letter explains how to distinguish hypothyroidism from depression and describes the treatment options for mood problems caused by an underactive thyroid.