Thyroid disorders are fairly commonplace in medical practice. Most people have either heard of or know an individual that has hyperthyroidism (e.g. Grave’s disease) or hypothyroidism (e.g. Hashimoto’s disease).
The thyroid gland is a butterfly-shaped gland in the throat. The main hormone it produces is thyroxine (T4). T4 is produced in large amounts, has a low potency, and converts to triiodothyronine (T3). T3 is produced in small amounts from T4 and has a high potency in the body. T4 has four iodine molecules attached to a tyrosine molecule, but T3 only has three (Figure 1). However, if the iodine molecules are arranged a different way, T3 becomes reverse T3 (rT3).
Some rT3 is always created in the body, but the conversion of T4 to rT3 can be increased due to factors such as stress (high cortisol), inflammation (pro-inflammatory cytokines), and dieting. rT3 is an inactive byproduct that blocks T3 receptors. Even if levels of T3 are adequate, they may not be able to exert their function if rT3 is blocking too many T3 receptors. Elevations in rT3 may compete with otherwise normal T3 and T4 levels and impair health.
Reverse T3 is a valuable marker for identifying hypothyroidism when traditional lab work shows normal T3 levels. Make sure to include rT3 in your thyroid assessments to insure you are getting the most complete evaluation of your patient’s symptoms.
van den Beld, AW, et al. (2005). Thyroid hormone concentrations, disease, physical function, and mortality in elderly men. J Clin Endocrinol Metab, 90(12): 6403-9.