SUBJECT: Your thyroid gland is found just below your voice box or larynx. It wraps around your windpipe or your trachea. Your thyroid affects your metabolism. It makes hormones that affect how fast your whole body works and how it uses energy. Your body uses thyroid hormone to increase your energy and raise your body temperature when needed. For example, that helps replace the heat your body loses when exposed to cold weather.
Thyroid hormone replacement has been used for more than a century to treat hypothyroidism. Natural thyroid preparations (thyroid extract, desiccated thyroid, or thyroglobulin), which contain both thyroxine (T4) and triiodothyronine (T3), were the first pharmacologic treatments available and dominated the market for the better part of the 20th century. Dosages were adjusted to resolve symptoms and to normalize the basal metabolic rate and/or serum protein-bound iodine level, but thyrotoxic adverse effects were not uncommon. Two major developments in the 1970s led to a transition in clinical practice: 1) The development of the serum thyroid-stimulating hormone (TSH) radioimmunoassay led to the discovery that many patients were overtreated, resulting in a dramatic reduction in thyroid hormone replacement dosage, and 2) the identification of peripheral deiodinase-mediated T4-to-T3 conversion provided a physiologic means to justify l-thyroxine monotherapy, obviating concerns about inconsistencies with desiccated thyroid. Thereafter, l-thyroxine mono-therapy at doses to normalize the serum TSH became the standard of care. Since then, a subgroup of thyroid hormone–treated patients with residual symptoms of hypothyroidism despite normalization of the serum TSH has been identified. This has brought into question the inability of l-thyroxine monotherapy to universally normalize serum T3 levels. New research suggests mechanisms for the inadequacies of l-thyroxine monotherapy and highlights the possible role for personalized medicine based on deiodinase polymorphisms. Understanding the historical events that affected clinical practice trends provides invaluable insight into formulation of an approach to help all patients achieve clinical and biochemical euthyroidism.
Megan Casper, RDN, a dietitian based in New York City and the founder of Nourished Bite, points out that iodine deficiency is the leading cause of hypothyroidism worldwide. This mineral can’t be made by the body, so dietary sources like iodized salt, dairy products, seafood, seaweed, and fortified cereals are important. “Iodine is an essential nutrient in the body, and thyroid hormones are composed of iodine,” explains Rizzo. “Those lacking thyroid hormones may also be lacking iodine.”
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