To regulate basal metabolic rate, the thyroid gland synthesizes hormones thyroxine, or triiodo-L-thyronine (T4), and the more active triiodo-L-tyronine (T3) (1).When secreted into the bloodstream, they associate with transport proteins (thyroxine-binding globulin, albumin, and transthyretin), which circulate the hormones (2). There is nearly 50 times T4 than there is T3, but T3 is more potent. T4 and T3 bind to nuclear receptors that affect gene expression. Increased mRNA and protein synthesis appear to lead to stimulation of oxygen consumption (when awake, at rest or fasting), heat production and also influences enzymes involved in carbohydrate metabolism (3;4).
1. Devlin TM. Textbook of Biochemistry with Clinical Correlations. Philadelphia: Wiley-Liss, 2002.
2. Gropper SS, Smith JL, Groff JL. Advanced Nutrition and Human Metabolism. Belmont, CA: Thomson Wadsworth, 2009.
3. Pereira BM, Balasubramanian K, Govindarajulu P. Effect of thyroxine treatment on epididymal carbohydrate metabolism in the pubertal rat. Int J Androl 1983;6:283-93.
4. Hashimoto K, Ishida E, Matsumoto S et al. Carbohydrate response element binding protein gene expression is positively regulated by thyroid hormone. Endocrinology 2009;150:3417-24.