The thyroid gland secretes hormones which regulate metabolic pathways, thus controlling various physiological functions. The thyroid gland works together with the hypothalamus and the pituitary gland to regulate the release of the different thyroid hormones.
First the hypothalamus releases Thyrotropin Releasing Hormone (TRH), which signals to the pituitary gland to release a dose dependent amount of the hormone Thyrotropin (TSH). Then the TSH stimulates the thyroid to produce Thyroxine (T4) and Triiodothyronine (T3) assuming the individual has the proper presence of Iodide. T3 and T4 are the metabolically active thyroid hormones that regulate diverse biochemical processes throughout the body. This is essential for normal development, as well as metabolic and neural activity. Failure at any level in any of the three glands of regulation will result in either hypothyroidism (under-active thyroid) or hyperthyroidism (over-active thyroid).
Doctors have been using blood tests to measure TSH and T4 levels as an assessment of the thyroid function for a very long time. If T4 levels are elevated, the diagnosis is classically hyperthyroidism; if they are depressed, hypothyroidism is diagnosed. The thyroid hormone levels are not consistent in the body, and can be changed by many factors, including pregnancy, increases in muscle mass, use of oral contraceptives and estrogen products, hepatitis and other diseases with altered hormones, major illness or large amounts of stress, to name a few. This is why it is so important to monitor thyroid levels on a consistent basis within individuals who possess a thyroid malfunction, and is also why no two cases are ever identical.