Hypothyroidism has been estimated to effect up to 10 percent of middle aged Americans and can produce depression. Some hypothyroidism sufferers are unable to convert inactive tetraiodothryronine (T4 ), ( levothyronine,Synthroid) to the active thyroid hormone triiodothryronine (T3), ( liothyronine, Cytomel). Many physicians assume patients who do not respond to low cost generic T4 are not hypothyroid, without attempting a trial of more expensive Cytomel. Frequently, Cytomel will improve hypothyroidism, when T4 medication does not. Cytomel is fast acting, with a short half-life and patients who have not improved with years of T4 medication, often feel better the same day Cytomel is taken. Bipolar disorder which fails to respond to more conventional Lithium or anticonvulsant medications sometimes responds well to T3 medication. Lithium medication can cause hypothyroidism.

The usual laboratory tests for thyroid hormone (TSH, T4, T3) sometimes fail to detect hypothyroidism, especially Hashimoto's autoimmune thyroiditis, which is becoming increasingly prevalent. More expensive fluorescence activated microsphere assay tests can sometimes detect this condition, where the immune system attacks thyroid tissue.

Jacob Teitelbaum, (2005) reports the American Academy of Clinical Endocrinologists narrowed the normal range for thyroid stimulating hormone (TSH), in November, 2003. A TSH of 3 or higher now indicates hypothyroidism. Since the older criteria considered hypothyroidism to begin with a TSH of 5, instead of 3.0, the number of hypothyroid individuals in the United States, is now estimated to have increased from 13 to 26 million. Teitelbaum states less than 25 percent of these people have been properly diagnosed and treated. Teitelbaum discusses the horrendous human cost of hypothyroidism, in Press Release: The Tragic and Invisible Epidemic of Thyroid Disease.