The thyroid gland, mainly through the secretion of thyroid hormone, has a wide ranging effect on the entire body. The gland itself takes the shape of a butterfly, and is found at the front of the trachea (windpipe), just below the larynx. The reason that thyroid hormone can affect so much of the body is that it is primarily responsible for regulating the body's metabolism. It controls the furnace, so to speak.

Thyroid hormone regulates the rate at which the body carries out its physiological functions, particularly in the area of fat and carbohydrate metabolism. The thyroid gland and thyroid hormone are bound up in a feedback loop that ultimately begins with the hypothalamus. We have seen how the hypothalamus was involved with the pituitary gland and levels of HGH, and a similar feedback loop occurs in the case of thyroid hormone as well. When the hypothalamus detects that the levels of thyroid hormone in the body are low, it releases thyroid releasing hormone (TRH). Thyroid releasing hormone directs the pituitary gland to secrete thyroid stimulating hormone (TSH), which in turn directs the thyroid gland to secrete thyroid hormone. As the biologically active thyroid hormone, triiodothyronine, is disseminated throughout the body, it stimulates the cells to proceed with the production of energy by means of burning fats and carbohydrates. As the thyroid hormone does its job it becomes depleted. When levels drop off sufficiently, this is detected by the hypothalamus and the cycle begins all over again.

The feedback loop we have just considered is, of course, an extreme oversimplification. We have already seen that the HGH feedback loop also affects levels of thyroid hormone (and others), and that many of the hormones affect each other, as well, in the manner of a complex Rube Goldberg machine. (Those of a certain age will recall the cartoonist's work, which invariably depicted an intricate set of contraptions wherein the initial one would set off the part it was hooked up to, and that one would in turn set off the next, and so on, until some final event was triggered.)