About the thyroid gland and thyroid hormones
The thyroid gland is a butterfly shaped gland which is found in the front of the neck. It is an important gland for controlling the rate of metabolism in the body. It also produces a hormone that helps to regulate the amount of calcium that circulates in the bloodstream.
What are the thyroid hormones and what do they do?
The thyroid gland produces 3 hormones; calcitonin, thyroxine (T4) and triiodothyronine (T3). T3 and T4 have similar functions and are involved in the control of metabolism, whereas calcitonin is involved in regulating the levels of blood calcium.
T3 and T4
T3 and T4 are often collectively referred to as the thyroid hormones. They are both made by follicular cells of the thyroid gland from the element iodine. Without sufficient iodine, the thyroid hormones cannot be produced which can lead to hypothyroidism. T3 is produced in smaller amounts than T4, but is more active than T4. Once inside its target cell, T4 is converted to the more active T3. T3 and T4 can act on nearly any cell of the body, where they have a number of roles. Thyroid hormones are especially important in children, as they are necessary for normal growth and development. Their main role is to control the body’s metabolic rate, which is essentially how much energy the body is using. T3 and T4 affect how we breakdown the main sources of fuel in the body – carbohydrates, fats and protein. They promote the production of heat by the body and also influence the rate of protein synthesis in cells. T3 and T4 work with growth hormone to promote growth of bones in children and adolescents, and also make the cells of the body more responsive to the hormone adrenaline.
Control of T3 and T4
The release and production of T3 and T4 is controlled by two glands in the brain; the hypothalamus and the pituitary. When the levels of T3 and T4 in the bloodstream decrease, or in situations of stress or extreme cold, the hypothalamus secretes a hormone, called thyrotropin releasing hormone (TRH) into a vein which travels directly to the pituitary gland. TRH causes the pituitary to release thyroid-stimulating hormone (TSH) which then travels to the thyroid gland via the bloodstream. TSH acts on the thyroid gland and causes the release of T3 and T4 into the blood, where the hormones are carried by proteins to their target tissues. T3 and T4 can act on numerous target tissues, such as the brain, bone, heart and the muscles and carry out the functions described above. When the blood levels of T3 and T4 return to normal, the hormones act on the hypothalamus and the pituitary gland to inhibit their release of TRH and TSH. This is called negative feedback.
Calcitonin
Calcitonin is produced by the C cells of the thyroid gland and is released when levels of calcium in the blood are increased. Its role is to decrease the amount of calcium in the blood. It has the opposite action to parathyroid hormone, which increases the amount of calcium in the blood when levels fall.
Control of Calcitonin
Calcitonin release is governed primarily by the level of calcium in the bloodstream. When calcium levels are high, more calcitonin is released from the gland. When calcium levels fall, the release of calcitonin also falls. Some other hormones play a role in the regulation of calcitonin. Somatostatin is a hormone which reduces the release of calcitonin, and gastrin promotes the release of calcitonin. Both gastrin and somatostatin primarily act in the gastrointestinal tract.
Calcitonin works to reduce the amount of calcium in the blood by acting on the bone, kidneys and the gastrointestinal tract. In the bone, calcitonin blocks the activity of a type of cell called an osteoclast. Osteoclasts break down bone and thus cause the release of calcium and phosphate into the bloodstream. By inhibiting osteoclasts, less calcium enters the bloodstream. Calcitonin also acts on the kidneys to increase the amount of calcium excreted in the urine and acts on the gastrointestinal system to limit the amount of calcium that is absorbed from the gut.
If you have questions about the thyroid gland contact your local doctor, who will arrange for you to see a thyroid surgeon.
