The thyroid gland is comprised of two lobes situated on the ventral surface of the trachea, in humans and other mammals. In many other vertebrates, the two lobes of the gland are separated on the two sides of the pharynx.
Hormones produced by the Thyroid Gland
Two very similar hormones are produced by the thyroid gland, both of which are derived from the amino acid tyrosine; triiodothyronine (T3) and thyroxine (T4). T3 contains 3 iodine atoms and T4 contains 4 iodine atoms, as their abbreviated terms suggest.
The thyroid gland secretes mainly T4 in mammals, but target cells convert most of it to T3, T3 having a greater affinity for the hormone receptor, situated in the nucleus of the cell. Accordingly, it is primarily T3 which triggers hormonal response in target cells.
The hypothalamus and pituitary gland carefully control the secretion of thyroid hormones via a complex negative feedback system.
Thyroid Hormones and Development
The thyroid gland plays an essential part in vertebrate development and maturation. In the metamorphosis of a tadpole into a frog, and all the reorganisation of many different tissues this process entails, the thyroid gland is of paramount importance. The thyroid gland is of equal significance in human development and growth. Cretinism, an inherited condition which occurs due to thyroid deficiency, results in severely retarded skeletal growth and poor mental development. Such defects can be at least moderately overcome if treatment with thyroid hormones is commenced early on in life. Studies, in non-human animals, have shown that optimum levels of thyroid hormones are vital and necessary for the normal functioning of bone-forming cells and for the branching of nerve cells throughout embryonic development of the brain.
Thyroid Hormones and Homeostasis
In homeostasis, the thyroid gland also plays a crucial role. For example, in adult mammals, thyroid hormones facilitate the maintenance of normal blood pressure, heart rate, muscle tone, digestion and reproductive functions.
Throughout the body, T3 and T4 are essential in bioenergetics, generally increasing the rate of oxygen consumption and cellular metabolism. Severe metabolic disorders can result from having either too much or too little concentration of the thyroid hormones in the bloodstream. In humans, if excessive thyroid hormones are secreted, a disorder known as hyperthyroidism occurs, producing symptoms such as elevated basal body temperature, profuse sweating, weight loss, irritability and high blood pressure. In contrast, the opposite condition caused by too little thyroid hormone, hypothyroidism, can cause cretinism in infants and produce symptoms such as weight gain, sluggishness and lethargy, and intolerance to cold in adults.
Another disorder related to a shortage of thyroid hormones is a swelling and enlargement of the thyroid gland, known as a goitre. Goitres are often caused by a deficiency of iodine in the diet. The thyroid gland is unable to produce adequate amounts of T3 and T4. Consequently, the pituitary continues to secrete thyroid stimulating hormone (TSH), thus leading to an enlargement of the thyroid gland.
Calcitonin and the Thyroid Gland
In addition to having cells that secrete T3 and T4, the thyroid gland of mammals contains endocrine cells which secrete calcitonin. This hormone reduces the level of calcium in the blood as part of calcium homeostasis.
There are four parathyroid glands which are embedded in the exterior surface of the thyroid, which function in calcium homeostasis. They secrete parathyroid hormone (PTH) which raises the levels of calcium ions in the blood, thus having an entirely opposite effect to calcitonin secreted by the thyroid gland. A lack of PTH causes levels of calcium in the blood to drop rapidly, leading to convulsive contractions of the skeletal muscles, known as tetany. Left untreated, this condition is fatal. Calcium ions, therefore, in the correct concentration, are essential to the normal functioning of all cells.
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