Bipolar disorder is a relatively new term that psychiatrists use to describe individuals whose emotions tend to swing from highs to lows, elation to the "blues," more than most other people. A sub-group of this population experience "rapid cycling," meaning that they have at least four major highs and lows per year. In studies of patients with rapid cycling bipolar disorder (85% of whom are women), 25-50% have been shown to have evidence of thyroid deficiency. Some feel well, and their only evidence of thyroid failure is an increased level of TSH in their blood. Others are clearly hypothyroid.
Physicians have also learned that lithium, a particularly effective drug in treating bipolar disorder, may reduce thyroid function and cause hypothyroidism in susceptible individuals, primarily those with a tendency to autoimmune thyroid disorders. Therefore, if you or someone in your family experiences these major mood swings, your physician may order tests for thyroid problems including a measurement of your blood level of TSH. If treatment with lithium is chosen, follow-up TSH blood tests from time to time are indicated. If your thyroid fails, you can continue lithium treatment and simply add thyroid hormone therapy to correct your thyroid deficiency.
Some physicians screen patients for a tendency to thyroid dysfunction by means of a blood test for antithyroid antibodies before they prescribe lithium treatment. It seems reasonable that those patients with a positive antibody test should have periodic TSH tests throughout the period of treatment with lithium. In reality, most physicians order occasional TSH tests on any patient taking lithium.