The symptoms of hypothyroidism may mimic depression, whereas hyperthyroidism may cause anxiety syndromes, mania, or depression. Thyroid function tests affected by depression include blunting of the thyoroid-stimulating hormone response to thyroid-releasing hormone stimulation and mild elevations in thyroid hormone levels (acute psychiatric admissions) that do not require antithyroid therapy because they resolve within 2 weeks without specific antithyroid therapy. Among thyroid antibody positive patients, an increased prevalence of rapid cycling bipolar disease has been reported. The nocturnal surge of thyroid-stimulating hormone is frequently absent among depressed patients but returns to normal with resolution of the depression or with sleep deprivation. Medications affecting thyroid function tests include lithium carbonate, which may cause hypothyroidism. Phenytoin (Dilantin(R)), carbamazepine (Tegretol(R)), phenobarbital, and valproic acid all reduce total thyroxine levels through enhanced liver clearance of free thyroxine and/or by displacement of the hormone from thyroxine binding globulin. Enhancement of antidepressant therapy with adjuvant 3,5,3'-triiodothyronine appears to be without obvious side effects and may accelerate the onset of the antidepressant response and/or induce responses in a subset of pharmacologically resistant subjects. However, a controlled multicenter trial is required to test this issue. It is prudent to check thyroid function tests in psychiatric patients who have an increased risk for thyroid disease, such as women over 45 years of age, patients with a family history, and those receiving lithium or suffering from dementia. L-thyroxine therapy is recommended for depressed subjects with concomitant evidence of subclinical hypothyroidism, especially with positive antithyroid antibodies.
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