This video describes the anatomy, and function of the thyroid and parathyroid glands, hormone functions and effects of imbalances.
THYROID AND PARATHYROID
The thyroid gland is located below the larynx. It consists of thyroid follicles composed of follicular cells that secrete the hormones thyroxine (T4) and triiodothyronine (T3), and parafollicular cells that secrete calcitonin (Figure 17.6). Follicular cells surround thyroid follicles, and the smaller parafollicular cells are located between the follicles.
Thyroid Hormones
Thyroid hormones are produced by the thyroid gland in response to stimulation by the anterior pituitary hormone TSH, and they are extremely important regulators of metabolism. TSH production is stimulated by exercise and stress so that energy is available to ATP production. They act on every cell in the body because all cells must make ATP. Triiodothyronine (T3) and thyronine (T4) are produced from the amino acid tyrosine and the trace mineral iodine (Figure 17.7). T3 contains three iodine atoms and T4 contains four iodine atoms. The thyroid gland is the only tissue in the body that requires iodine. Selenium is also a very important micronutrient required for thyroid hormone synthesis, and selenium deficiency can cause severe thyroid malfunction. This micronutrient is found in foods such as nuts (especially Brazil nuts), fish, wheat bran, pork, shellfish, eggs, and mushrooms. The follicular cells of the thyroid gland produce both T3 and T4. However, once these hormones circulate in the blood and are taken up by cells throughout the body, most T4 is converted into T3, which is the more active form. Although thyroid hormones are produced from the amino acid tyrosine, they are not water soluble and must be transported through the blood on plasma proteins. Once they reach a target cell, they enter the cell through a membrane transporter protein and act on the receptors inside the cell that affect gene transcription.
Did You Know?
Table salt is “iodized” because it contains iodine in order to prevent deficiencies that can lead to hypothyroidism. Before iodization of salt, millions of people in the world had severe iodine deficiency that caused goiter. Iodine is common in soils located near oceans, but many inland regions lack iodine. In the early 1920s, iodine was routinely added to table salt in North America, and thyroid disease dramatically decreased.
The thyroid hormones have the following functions in the body:
- Regulating oxygen use by all cells during cellular respiration
- Increasing basal metabolic rate (BMR) by increasing the production of ATP from glycogen and fats
- Increasing the synergistic effects of epinephrine and norepinephrine in the breakdown of glycogen and fat for energy production
- Stimulating the production of growth hormone, especially important during fetal development
- Increasing heart rate, breathing rate, and cardiac output so that more oxygen can be delivered to cells
Abnormal Functioning of the Thyroid Gland
Hypothyroidism is a condition caused by insufficient production of thyroid hormones. People with hypothyroidism are not able to utilize nutrients to produce energy, and so they feel very tired, have low energy, often gain weight, feel cold (since body heat is produced when ATP is produced), often have dry skin and brittle nails, and sometimes experience depression. Thyroid hormones play a role in the production of serotonin, and a lack of serotonin leads to depression. The most common causes of hypothyroidism are iodine deficiency, selenium deficiency, any malfunction of the hypothalamus or anterior pituitary gland, extreme stress, and anorexia. Note that very low-calorie diets cause the body to stop producing thyroid hormones; if nutrients aren’t available for energy production, the body won’t produce the necessary hormones to break down nutrients to produce energy. Eating properly and exercising regularly helps ensure sufficient thyroid hormone production.
Hashimoto’s disease is an autoimmune disease in which the immune cells destroy the thyroid cells. The lack of thyroid hormone stimulates an increase in TSH secretion from the anterior pituitary gland and this can lead to goiter. Goiter can occur in cases of both hypothyroidism and hyperthyroidism.
Hyperthyroidism is a condition caused by excessive production of thyroid hormones. People with hyperthyroidism feel nervous, irritable, hot; have increased heart rate and difficulty sleeping; usually have weight loss; and sometimes have protruding eyes (exophthalmos) from the swelling of the fat and muscle tissue behind the eyes. The most common cause of hyperthyroidism is Graves’ disease, which is an autoimmune disease (Figure 17.8). Antibodies that act against the thyroid-stimulating hormone (TSH) bind to the TSH receptors instead of TSH, and they continually stimulate the thyroid to produce thyroid hormones, which causes the thyroid gland to enlarge (goiter), and further increases thyroid hormone production.
Calcitonin
Calcitonin (CT) is the hormone produced by the parafollicular cells of the thyroid gland. The main function of calcitonin is to prevent calcium (Ca2+) blood levels from being too high; it balances the effects of parathyroid hormone, which increases blood calcium levels. Calcitonin secretion from the parafollicular cells is regulated by calcium blood levels. When levels increase, calcitonin is produced and inhibits osteoclast activity. Osteoclasts are the bone cells that break down the mineral matrix and release stored calcium and phosphate into the blood (Chapter 13).
The functions of calcitonin include the following: inhibiting osteoclast activity, inhibiting calcium absorption in the small intestine, and inhibiting reabsorption of calcium in the kidney. Because it prevents bones from losing calcium, calcitonin can be used to treat osteoporosis.
Parathyroid Hormone
Parathyroid hormone (PTH) is produced by the chief cells in the parathyroid glands, which are located on the posterior side of the thyroid gland, and is the major regulator of blood calcium (Ca2+), magnesium (Mg2+), and phosphate ions (PO43–) in the blood (Figures 17.9 and 17.10).
The functions of PTH include the following:
- Stimulating osteoclasts to release calcium and phosphate into the bloodstream
- Stimulating kidney cells to increase reabsorption of Ca2+ and Mg2+
- Increasing the excretion of PO43– by the kidneys
- Increasing the production of active vitamin D (calcitriol) by the kidneys, which then increases the absorption of calcium in the small intestines
PTH is stimulated by low blood calcium levels. Very low calcium levels caused by calcium deficiency can cause hypersecretion of PTH, which causes excessive bone demineralization and osteoporosis. Calcium deficiency can be caused by insufficient dietary intake of calcium, vitamin D deficiency, or renal failure. Foods high in calcium include dairy products, such as milk, cheeses and yogurt; leafy green vegetables; fish; beans; and nuts.
