Thyroid hormones regulate metabolism. The U.S. National Library of Medicine's Medline Plus web site tells us that millions of Americans have thyroid disease, causing either too much or too little production of these hormones. Women are affected more often than men.
Hypothyroidism is the condition characterized by too little thyroid activity, with symptoms of weight gain, fatigue, and sensitivity to cold temperatures. Hyperthyroidism is the condition characterized by too much thyroid activity, with symptoms of weight loss, accelerated heart weight, and sensitivity to heat. Hypothyroidism is the more common one.
Dr. Graham R. Williams and Dr. J. H. Duncan Bassett of Imperial College London, UK, provided a review of thyroid hormones. The thyroid produces primarily T4, also called tetraiodothyronine or thyroxine. The name comes from the four iodine atoms in the molecule. T4 is actually a pro-hormone, which is not physiologically active.
Deiodinase enzymes convert T4 to the active hormone T3, also called triiodothyronine, by removing one iodine atom. A similar enzyme inactivates T3 by removing another iodine atom to form T2. The thyroid also produces small amounts of T3.
Hypothyroidism is treated by replacement T4 given orally. However, the deiodinase enzymes may be just as important, according to Williams and Bassett. “Ultimately, an important challenge will be to exploit DIO2 [a deiodinase enzyme] as a drug target to manipulate tissue thyroid status, perhaps in the treatment of metabolic disorders including obesity or skeletal disorders such as osteoporosis and osteoarthritis,” they wrote.
Williams and Bassett explained that the production of T4 is controlled by thyrotropin-releasing hormone (TRH), produced by the hypothalamus, and by thyroid stimulating hormone (TSH), produced by the pituitary gland. TSH is the most commonly measured indication of thyroid function.
When T3 and T4 levels in the blood drop, the pituitary produces more TSH, which is a signal to the thyroid to produce more T4. When T3 and T4 levels rise, the pituitary produces less TSH.