Thyroid And Parathyroid Glands

The thyroid gland is an endocrine gland located anterior to the trachea and inferior to the thyroid cartilage. It is butterfly shaped with three lobes that make it the largest endocrine organ in the body. Its primary function is to control the body’s metabolism by regulating how quickly the body uses energy, proteins, and hormones.

The two lateral lobes are connected by the middle lobe called the isthmus, which lies directly on top of the trachea. The entire gland is enclosed in a capsule made up of connective tissue called the thyroid cartilage. The thyroid has an irregular surface configuration and is highly vascular with a liberal blood supply. Thyroid tissue is made up of columnar and cuboidal cells that form follicles, which produce thyroid hormones. The follicle cavities are filled with fluid called colloid. The thyroid receives blood from branches of the external carotid artery and the brachiocephalic trunk, and it drains blood via the internal jugular vein and left brachiocephalic vein.

When the pituitary gland releases Thyroid-Stimulating Hormone (TSH), two thyroid hormones are released: Triiodothyronine (T3) and Thyroxine (T4). These two hormones target muscle cells and cause an increase in metabolism. Generally, the majority of T4 is converted to T3 by peripheral organs such as the liver, kidney and spleen. The lobes also contain parafollicular cells that produce calcitonin, which inhibits calcium secretion and stimulates osteoclasts when blood calcium levels are high. The thyroid is a unique gland in that it is the only gland that both stores and secretes hormones. It is also the only gland that is dependent on dietary habits to function due to its need for iodine—the hormones it secretes are synthesized from iodine and tyrosine. Triiodothyronine contains three atoms of iodine, and thyroxine contains four atoms of iodine.

The most common problems of the thyroid gland are hyperthyroidism, which is an overactive thyroid gland, and hypothyroidism, which is an underactive thyroid gland. Hyperthyroidism is considered an autoimmune disease due to the antibodies that stimulate the thyroid to secrete excessive quantities of hormones. Common symptoms include a thyroid goiter, cardiac palpitations, weight loss, and increased appetite. The opposite problem would be considered hypothyroidism, which is commonly characterized by symptoms such as abnormal weight gain, fatigue, cold intolerance, and bradycardia. Another disease that targets the thyroid gland is cancer. It most often presents as a painless mass in the neck and does not produce symptoms until it is advanced.

On the posterior side of the thyroid, between the two layers of the capsule on both sides there are two parathyroid glands. The parathyroid glands are two paired glands located behind the thyroid gland in four oval structures that primarily function to maintain the body’s blood calcium level. They are round masses of tissue enclosed in a capsule made up of connective tissue. The parathyroid tissue is made up of glandular epithelial cells: more specifically, chief cells and oxyphil cells. While they are structurally and functionally separate from the thyroid gland, the parathyroid glands are often difficult to differentiate from thyroid or fat using the naked eye.

The chief cells produce parathyroid hormone (PTH), which is a small protein that stimulates calcium secretion and inhibits osteoclasts when blood calcium levels are low. It has the opposite effect of calcitonin.

Image Caption: Thyroid And Parathyroid Glands. Credit: Wikipedia