Thyroxine, or T4, is the prohormone of the active hormone triiodothyronine, or T3. The thyroid gland produces T4 and T3 at a ratio of 4 to 1, notes Endocrineweb. Thyroxine provides a circulating reservoir of active triiodothyronine. It is partly active at target tissues although it is approximately four times less potent than T3. The majority of T4 is rapidly converted to T3 in target cells. The primary function of thyroid hormones is to regulate basal metabolism, which is what occurs during rest and not exercise.
Energy Production
Basal metabolism refers to the amount of energy required by the vital organs, such as heart, lungs and nervous system, to function while at rest. T3 stimulates basal metabolism and increases oxygen consumption and energy and heat production.
Glucose Metabolism
T3 stimulates all aspects of glucose metabolism, notes the Colorado State University website. It increases breakdown of glycogen, which is a molecule that stores glucose, it stimulates liver glucose production and it increases glucose uptake by all cells in the body. It also potentiates the action of insulin, the primary hormone that regulates glucose metabolism.
Protein Metabolism
T3 stimulates both protein degradation and protein synthesis; however it more extensively increases protein degradation, especially in muscle. High levels of thyroid hormone are associated with muscle wasting and decreased muscle strength. In children low levels of thyroid hormone can impair growth because of its affects on protein metabolism.
Fat Metabolism
T3 increases fat and cholesterol breakdown and enhances their utilization by other tissues. A common feature of a person with low levels of thyroid hormone is increased levels of circulating lipids and cholesterol. A 2002 review in the journal, "Hormones" notes that on the other hand that serum cholesterol and lipid levels are reduced in people with hyperthyroidism.
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