- Describe the role of the thyroid and parathyroid glands in the endocrine system
The thyroid gland is located in the neck, just below the larynx and in front of the trachea, as shown in Figure 1. It is a butterfly-shaped gland with two lobes that are connected by the isthmus. It has a dark red color due to its extensive vascular system. When the thyroid swells due to dysfunction, it can be felt under the skin of the neck.
The thyroid gland is made up of many spherical thyroid follicles, which are lined with a simple cuboidal epithelium. These follicles contain a viscous fluid, called colloid, which stores the glycoprotein thyroglobulin, the precursor to the thyroid hormones. The follicles produce hormones that can be stored in the colloid or released into the surrounding capillary network for transport to the rest of the body via the circulatory system.
Thyroid follicle cells synthesize the hormone thyroxine, which is also known as T4 because it contains four atoms of iodine, and triiodothyronine, also known as T3 because it contains three atoms of iodine. Follicle cells are stimulated to release stored T3 and T4 by thyroid stimulating hormone (TSH), which is produced by the anterior pituitary. These thyroid hormones increase the rates of mitochondrial ATP production.
A third hormone, calcitonin, is produced by parafollicular cells of the thyroid either releasing hormones or inhibiting hormones. Calcitonin release is not controlled by TSH, but instead is released when calcium ion concentrations in the blood rise. Calcitonin functions to help regulate calcium concentrations in body fluids. It acts in the bones to inhibit osteoclast activity and in the kidneys to stimulate excretion of calcium. The combination of these two events lowers body fluid levels of calcium.
Most people have four parathyroid glands; however, the number can vary from two to six. These glands are located on the posterior surface of the thyroid gland, as shown in Figure 2. Normally, there is a superior gland and an inferior gland associated with each of the thyroid’s two lobes. Each parathyroid gland is covered by connective tissue and contains many secretory cells that are associated with a capillary network.
The parathyroid glands produce parathyroid hormone (PTH). PTH increases blood calcium concentrations when calcium ion levels fall below normal. PTH and calcitonin work in opposition to one another to maintain homeostatic Ca2+levels in body fluids. These hormones encourage bone growth, muscle mass, and blood cell formation in children and women. PTH is produced by chief cells of the parathyroid. Another type of cells, oxyphil cells, exist in the parathyroid but their function is not known.