Speeding Up With Thyroid Hormones – The Effect of Thyroid Hormones on Metabolism
The thyroid hormones are known to be able to increase the metabolic rates of many tissues in the body. These hormones are produced by the thyroid gland, a small, butterfly-shaped gland located in the front of the neck directly in front of the trachea. This gland consists of the left and the right lobe, which are both connected by a small connecting branch or the isthmus.
The thyroid is unique among the other endocrine glands in that it requires a trace element in the production of its hormones. This trace element is iodine. Another unique feature of the thyroid gland is its ability to store the hormones it produces inside follicles whereas the other endocrine glands produce the hormones and secrete them immediately.
These hormones affect the body’s tissues in several ways but most notably, they increase the metabolic rates of these many tissues. Let us discuss the effect of these specific hormones on carbohydrate, lipid, and protein metabolism.
Carbohydrate Metabolism
All aspects of carbohydrate metabolism are increased by thyroid influence, including glucose production by the liver particularly through gluconeogenesis. Gluconeogenesis is the production of glucose by the liver from stored fats or proteins. This effect does not increase plasma glucose concentrations because the pancreas are also stimulated by the hormone to secrete increased amounts of insulin to keep up with increased glucose production.
The hormone also enhance the availability of the materials required for gluconeogenesis. Specifically, this is done by increasing the expression of key enzymes of gluconeogenesis like phosphoenolpyruvate carboxyinase, pyruvate carboxylase, and glucose-6-phosphatase.
Fat Metabolism
Like carbohydrates, all aspects of fat metabolism are increased by the presence of increased concentrations of the hormone in circulation. Lipids, in particular, are rapidly mobilized from surrounding adipose tissues, decreasing the fat stores of the body more than any other tissue element. The glycerol required for the increased gluconeogenesis comes from these surrounding tissues.
The hormones also enhance lipogenesis in addition to lipolysis. Only very small amounts of hormone are necessary to maintain the normal synthesis of free fatty acids by the liver. Very high levels of hormone can cause the balance to shift in favor of lipolysis, which can cause depletion of fat stores.
The cycle of synthesis and degradation proceeds to such an extent that it becomes a futile cycle. This contributes to the increased oxygen consumption often seen in thyrotoxicosis.
Protein Metabolism
The amino acids required for continued hepatic gluconeogenesis are also derived from proteolysis, particularly muscle proteins. Thyroid hormones also increase protein synthesis but the degradation of protein usually outweighs the synthesis so there is still net loss of muscle proteins. This is particularly observed in cases of excessive triiodothyronine production wherein muscle wasting and weakness occur as well as increased nitrogen loss through the urine as urea.
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