The thyroid glandsecretes hormones which are necessary for normal growth and development from foetal life onwards, and for maintenance of normal metabolism in the adult body.
The gland is located just below the larynx and attached to the front of the trachea. The adult gland weighs 10-20g and consists of two relatively flat oval lobes linked by an isthmus. It is so named because of its resemblance to the classical shield used by the ancient Greeks. However, unlike the shield, in any individual the thyroid is generally asymmetric, with the right lobe being significantly larger than the left one. The gland is usually larger in women than in men and it increases slightly in size during pregnancy. This is exploited as an early pregnancy test in some African communities: the neck of a bride is adorned with a tight necklace and pregnancy is indicated when in due course the necklace is broken by the swelling thyroid gland.
The major functional and structural unit of the thyroid is the thyroid follicle. The unique biochemical characteristic of thyroid follicular cells is their ability to concentrate and to utilize dietary iodide. The thyroid is unique in its ability to retain and utilize the iodide for the biosynthesis of its hormones. These hormones are small molecules derived from the amino acid tyrosine and they have iodine incorporated into their structures.
There are two thyroid hormones, which have either 3 or 4 atoms of iodine per molecule; they are known respectively as T3 (triiodothyronine) and T4 (thyroxine). Both are synthesized within the thyroid follicles and secreted into the bloodstream when the cells are stimulated to extrude them by the thyroid stimulation hormone (TSH) from the pituitary gland. The thyroid hormones in the circulation in turn regulate the production of TSH by the pituitary, switching off TSH production when the appropriate level of T3/T4 is attained in the blood.
Following the accumulation of iodine in the follicular cells, the T3 and T4 are first synthesized separately and are then incorporated into a much larger molecule known as thyroglobulin. TSH stimulates the release of the T3 and T4 from the thyroglobulin so that the hormones can be secreted from the cells into the bloodstream in a regulated fashion. This hormone storage system is unique in endocrine physiology; it ensures that there is a two-month supply of thyroid hormones in the event that a person encounters an iodine deficient environment. This occurs in many parts of the world, such as some mountainous regions in China and India.
However, this capacity to store thyroid hormones within the follicles as thyroglobulin becomes disadvantageous if an individual inadvertently ingests radioactive iodine. This occurred after the huge release into the atmosphere of radioactive isotopes, including radioiodine, during the week following the Chornobyl accident on 26 April 1986. The natural storage of the radioiodine in the follicles delays clearance of the ingested radionuclide and concentrates the damaging radiation on the thyroid. In Ukraine and Belarus this resulted in a major increase in the incidence of thyroid cancer in the 1990’s among children born before the accident.
Thyroid hormones regulate the activities of almost all cells in the body. They exert three main classes of action. Firstly they control the basal metabolic rate (BMR). Secondly they influence cell differentiation and growth. Thirdly they may modify the action of other hormones, extending their importance still more widely.
Thus a lack of thyroid hormones is manifested in diverse ways. In the developing foetus an inadequate supply leads to impaired brain development with the danger of the infant being born a cretin. In an adult there is a depressed BMR with attendant lethargy. By contrast, excess thyroid hormones raise the BMR and may lead to cardiac problems due to potentiation by thyroid hormone of the effects of adrenaline.
It should also be mentioned that there are four tiny parathyroid glands located around the thyroid in the throat. “Para” means “near”, which explains the name. The main function of the parathyroid glands is to make the parathyroid hormone (PTH). This chemical regulates the amounts of calcium, phosphorus and magnesium in the bones and blood. When calcium levels in the blood are too low, the parathyroid glands release extra PTH, which leeches calcium from the bones and stimulates calcium reabsorption in the kidney. On the other hand, if the level of calcium in the blood is too high, the glands drop hormone production. Problems can occur if the parathyroids are overactive or underactive.