The thyroid is butterfly-shaped gland located in the front of the neck that regulates the body's metabolism. The right and left lobes of the thyroid are connected in the middle by a region called the isthmus. Within the thyroid, cells are grouped into clusters called follicle cavities. A follicle cavity is a fluid-filled, iodine-rich cavity in the thyroid surrounded by a single layer of follicular cells in which triiodothyronine and thyroxine are produced. They have a plentiful blood supply, nerve innervation, and lymph fluid. Follicle cavities are filled with a sticky fluid called colloid that contains thyroglobulin, the precursor to triiodothyronine and thyroxine. Triiodothyronine (T3) is a thyroid hormone containing three iodine ions that targets cells in the body to increase the metabolic rate. Thyroxine (T4) (also known as tetraiodothyronine) is a thyroid hormone containing four iodine ions that targets cells in the body to increase the metabolic rate. Iodine is necessary for the production of these hormones within the follicle cavity. To produce T3 and T4, enzymes in the follicle cavity attach iodine ions to the hormone precursor thyroglobulin. Thyroglobulin anchors T3 and T4, keeping them bound in the follicle cavity. When stimulated by thyroid-stimulating hormone (TSH), follicular cells take up this iodinized thyroglobulin via a process called endocytosis. Lysosomal enzymes digest the thyroglobulin, releasing free T3 and T4, which can diffuse across cell membranes and enter the bloodstream.
The thyroid hormones T3 and T4 have wide-ranging actions in the body. They act on all cells to increase the basal metabolic rate. They bind receptors on mitochondria, where they cause an increase in nutrient breakdown, oxygen use, and adenosine triphosphate (ATP) production. This process increases the amount of heat generated by the body. These hormones also increase heart rate, the rate of respiration, and the rate of nutrient uptake by the digestive system. In addition, T3 and T4 are critical for protein synthesis and normal fetal and childhood development and growth as well as adult neurological function. They also interact with reproductive hormones and play a role in the regulation of libido and fertility. Thyroid hormones provide negative feedback to the hypothalamus and anterior pituitary gland, moderating their release.
Triiodothyronine and thyroxine are not the only hormones produced by the thyroid. Parafollicular cells (also called C cells) are located in thyroid tissue between follicles. These cells respond to high levels of calcium in the blood to produce the hormone calcitonin. Calcitonin decreases the activity of osteoclasts, the cells in bones responsible for the breakdown of bone tissue to release calcium into the blood.
The parathyroid is a gland located behind each lobe of the thyroid that regulates calcium levels. There are usually four parathyroid glands in the human body, with one pair present behind each lobe of the thyroid. They are approximately the size of a grain of rice. They share a blood supply and lymphatic tissue with the thyroid. Along with the parafollicular cells of the thyroid, the parathyroid regulates calcium levels and maintains calcium homeostasis. Appropriate calcium concentration is vital for normal muscular and nervous system function and therefore must be tightly regulated by the body.The parathyroid glands are highly vascularized. This allows cells in the parathyroid to detect small changes in calcium concentration in the blood. If calcium levels are too low, the parathyroid gland produces and releases parathyroid hormone. Parathyroid hormone (PTH) stimulates osteoclasts (the cells in the bone that break down bone tissue) to increase the levels of calcium in the blood. This action is countered by the action of calcitonin, a hormone released from the thyroid that inhibits osteoclasts and decreases blood calcium. Another way PTH increases blood calcium levels is by increasing the activation of vitamin D in the intestines. This increases calcium absorption. Parathyroid hormone also acts on the kidneys, where it promotes calcium reabsorption into the bloodstream.