Impact of the Thyroid Function

The thyroid gland is the largest endocrine gland in your body. The primary thyroid function is to manufacture two hormones: thyroxine and triiodothyronine. Thyroxine is commonly called T4 because it contains four atoms of iodine; triiodothyronine is also known as T3 because of its three iodine atoms content. The thyroid secretes these two hormones into the bloodstream, which transports them around the body.

Thyroid functions affect the following:

Rate of Metabolism. The thyroid function controls the metabolism in all body tissues save for the testes, spleen, lungs and retina. When large quantities of T4 hormone are present, the body’s basal metabolic rate can rise by 60% to 100% above normal. If that happens, the rate of usage of glucose, fat, and proteins also increases.

Fats originating from lipids in adipose tissue are mobilized, and cholesterol catabolism by the liver increases. Thus you see blood cholesterol levels decrease in hyperthyroidism and rise in hypothyroidism. Muscle proteins are taken apart and used as fuel, which may explain the muscle fatigue associated with hyperthyroidism. Glucose absorption from the gastrointestinal tract is increased. Increased metabolic rate also causes higher utilisation of vitamins, thus tending to result in vitamin deficiency.

Cardiovascular and Respiratory Functions. Thyroid function strongly affects the functions of your cardiovascular and respiratory organs. Increased metabolism leads to higher oxygen consumption and a rise in vasodilatation. Blood flow to the skin, in particular, is augmented to help dissipate the body heat generated by higher metabolism.

Oxygen delivery to tissues must be increased, so cardiac output goes up, as will blood volume and ventilation. Heart rate increases to maintain the needed cardiac output. But, blood pressure is not likely to change much because the bigger vasodilatation (widening of blood vessels) offsets the increased cardiac output.

Gastrointestinal Secretions. Proper thyroid function enhances gastrointestinal function, but more active thyroid function will increase motility and production of digestive secretions that may result in diarrhoea. Higher metabolic rate triggered by increased thyroid hormones will cause an increased appetite and food intake. But, weight loss will often occur because of the higher utilisation of calories.

Neuromuscular Effects. The thyroid function can visibly affect the neural control on muscle functions and tone. Skeletal muscles react vigorously to even slight increases in thyroid hormone levels; decreases in hormone levels make them react sluggishly. In hyperthyroidism, you may observe a fine muscle tremor. The cause of the tremor is not understood, but it may mean the neural synapses in the spinal cord, which control muscle tone, have become more sensitive, as a result of an abnormal thyroid function.