The thyroid gland is a butterfly shaped gland in the throat (see a picture). The thyroid uses the mineral iodine to produce hormones that are involved in the regulation of a number of physiological processes, including metabolism and calcium balance. The importance of a properly functioning thyroid is shown by the disorders caused by the disrupted regulation of its hormones.
The three main hormones produced by the thyroid gland are thyroxine (T4), triiodithyronine (T3), and calcitonin. Production of T3 and T4 by the thyroid gland is triggered by thyroid stimulating hormone, which is produced by the pituitary gland. T3 most notably affects body temperature and heart rate, T4 most notably affects metabolism, and both affect growth. The hormones are particularly important in the growing bodies of children and thyrocalcitonin, works with parathyroid hormone to regulate calcium balance. Calcitonin promotes bone formation, lowering the calcium and phosphorus levels in the blood by blocking the function of osteoclasts (bone cells that are involved in bone removal) and promoting the function of osteoblasts (cells that lay down new bone).
See animations of the thyroid gland releasing hormones at Biology in Motion.
The importance of thyroid hormone regulation
Overactive thyroid, meaning that too much hormone is released into the bloodstream, is hyperthyroidism. This is sometimes called thyrotoxicosis. The symptoms of an overactive thyroid include excessive sweating, heat intolerance, rapid heart rate, nervousness and agitation, fatigue, loss of concentration, and changes in bodily functions. If left untreated, a “thyroid storm” can lead to heart failure and delirium.
Underactive thyroid, meaning that too little hormone is released into the bloodstream, is hypothyroidism. A lack of thyroid hormone slows down metabolism. The symptoms of hypothyroidism include cold sensitivity, depression, fatigue, constipation, muscle pain, and weakness. If untreated, the lack of thyroid hormone can lead to hoarseness, hair loss, slow speech, and decreased sense of taste and smell.
Calcitonin increases bone density in patients with osteoporosis, reducing the risk of fracture. The hormone is also given in Paget’s disease to reduce bone turnover. A lack of calcitonin production can result in hypercalcemia, which has a number of symptoms, including appetite suppression, thirst, fatigue, and muscle weakness, but more importantly, it can result in kidney stones and other renal complications. Heart electrical conductance can also be interrupted by hypercalcemia, making the thyroid gland important to regular body functions.
The involvement of the thyroid in metabolism and calcium balance makes it an essential gland. Its hormones touch nearly every aspect of system regulation in the human body.