Thyroid and parathyroid glands regulate various metabolic reactions in the human body. The thyroid gland located in the neck has two lobes interconnected by the isthmus. The thyroid gland is made up of follicles and stromal tissues. The follicular cells produce two hormones namely, triiodothyronine or T3, tetraiodothyronine or thyroxin or T4, whereas the parafollicular cells scattered between the follicles and the stroma produce thyrocalcitonin or TCT. The synthesis and secretion of T3 and T4 is regulated by the thyroid-stimulating hormone or TSH produced by the anterior pituitary gland.
Thyroid hormones regulate tissue growth and development, formation of red blood cells, metabolism of proteins, carbohydrates and fats, water and electrolyte balance, and the basal metabolic rate. For the normal rate of synthesis of thyroid hormones, the presence of iodine in the diet is a must. Deficiency of iodine in our diet leads to simple goitre, characterised by the enlargement of the thyroid gland. In pregnant women, hypothyroidism, can lead stunted growth in children called cretinism, characterised by mental retardation. It can also be caused due to the absence of iodine in the diet. Hypersecretion of thyroid hormones occurs due to cancer or development of nodules in the thyroid glands which leads to Graves’ disease or exophthalmic goitre in adults. It is characterised by an enlarged thyroid gland, protrusion of the eyeballs etc.
Hyposecretion of thyrocalcitonin causes osteoporosis, characterised by increasing calcium deposition in the bones.
There are two pairs of parathyroid glands partially embedded in the thyroid lobes dorsally, which secrete parathyroid hormone or PTH. The circulating levels of calcium ions regulate the secretion of PTH. PTH increases the activity of osteoclasts that results in elevated bone resorption, which releases ionic calcium and phosphates into the blood. PTH even increases the level of calcium in the blood, by stimulating the reabsorption of calcium by the renal tubules and increases calcium absorption from digested food. Hyposecretion of the parathyroid hormone causes parathyroid tetany, characterised by muscle spasms, contraction of the muscles of the face, hands, feet etc. Hypersecretion of the parathyroid hormone causes ostetitis fibrosa cystica, characterized by replacing bones with cysts and fibrous tissues. Hypersecretion of parathyroid hormone also produces stones in the kidneys and ureters.