The Role of the Pituitary Gland and Thyroid-Stimulating Hormone
The pituitary gland is situated at the base of the skull and is, without question, the most infl uential gland in your body. It is often referred to as the master gland. The pituitary gland acts as the body’s “thermostat” by sending out many types of stimulating hormones to the various parts of our bodies that make hormones.
Your thyroid gland reports directly to the pituitary gland, which monitors T4 and T3 levels in your body. When levels are low, it secretes TSH; this signals the thyroid gland to make thyroid hormone.
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The TSH stimulates the thyroid to take up iodine from the blood and make thyroid hormone, just as a thermostat’s electrical signal to the furnace stimulates it to take up fuel and make heat. When the thyroid hormone level in the blood rises to the proper level, it causes the pituitary to reduce its release of TSH, just as a thermostat turns off its electrical signal to the furnace. This regulates the thyroid hormone in the blood so that it stays at the proper level, in the same way the thermostat keeps a house at the proper temperature.
If the pituitary gland is working properly, a high level of TSH means that there is not enough thyroid hormone in the blood; a low level of TSH means that there is too high a level of thyroid hormone in the blood; and a normal level of thyroid hormone results in a normal level of TSH. The TSH level in the blood can be measured to tell whether the thyroid is making enough, too little, or too much thyroid hormone. The TSH test is the most accurate and sensitive blood test that exists for thyroid patients.
The Role of the Hypothalamus
The hypothalamus is a part of the brain located just above the pituitary gland. It is connected to the pituitary by a thin stalk that carries hormones that help control the pituitary. A part of the hypothalamus also works like a thyroid hormone thermostat, releasing its own signal, thyrotropin-releasing hormone (TRH), or TSH-releasing hormone, to the pituitary when the thyroid hormone levels are low. Thus there is a “double-thermostat” control of thyroid hormone in the body, although this can break down when something is wrong with the hypothalamus.
The Role of Calcitonin
In the thyroid gland, the cells that produce thyroid hormone are called follicular cells. Thyroid follicles are contained within the capsule of the thyroid gland like bunches of microscopic grapes. Between neighboring thyroid follicles are tiny blood vessels, lymph vessels, and collections of other cells, called parafollicular cells. These parafollicular cells, or C-cells, make additional hormones, such as calcitonin and somatostatin.
Calcitonin helps to regulate calcium and therefore helps to prevent osteoporosis. But to your bones, calcitonin is kind of like a tonsil. It serves a useful purpose, but when the hormone is not manufactured due to the absence of a thyroid gland (if it’s removed or ablated by radioactive iodine), you won’t really notice any effects, just as you don’t miss your tonsils. Calcium levels are really controlled by the parathyroid glands and are much more dependent on Vitamin D, which helps with calcium absorption; diet; and exercise, which builds bone mass.
Calcitonin is only important when discussing the thyroid in the context of a rare type of thyroid cancer known as medullary thyroid cancer. When this type of thyroid cancer develops, your thyroid overproduces calcitonin, which is a marker for medullary thyroid cancer.