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It is situated on the front side of the neck at the level of C5 and T1 vertebral bodies, just below the Adam's apple, near the thyroid cartilage over the trachea but covered by layers of skin and muscle. The thyroid is quite large for an endocrine gland - 15-30 grams in adults- and butterfly-shaped: the wings correspond to the lobes and the body to the isthmus of the thyroid. Normally it is larger during menstruation and in pregnant women.
The primary function of the thyroid is production of hormones:
The production of these hormones is regulated by thyroid-stimulating hormone (TSH), released by the pituitary.
The gland is composed of spherical follicles that selectively absorb iodine (more accurately iodide ions, I-) from the bloodstream and concentrate it for production of thyroid hormones. The follicles are made of a single layer of thyroid epithelial cells, which secrete T3 and T4. Inside the follicles is a colloid which is rich in a protein called thyroglobulin. It serves as a reservoir of materials for thyroid hormone production and, to a lesser extent, a reservoir of the hormones themselves. The spaces between the thyroid follicle spheres are filled with the other type of thyroid cells, parafollicular cellParafollicular cells also called C cells are cells in the thyroid which produce and secrete calcitonin. Hormone secreting cells.s or C cells, which secrete calcitonin.
In areas where iodine - essential for the production of thyroxine - is lacking in the diet, the thyroid gland can be considerably enlarged, resulting in the swollen necks of endemic goitreA goitre (or goiter) ( Latin struma is a swelling in the neck due to an enlarged thyroid gland. The most common cause for goitre in the world is iodine deficiency. Other causes are: Hashimoto's thyroiditis Graves-Basedow disease juvenile goitre neoplasm o.
Thyroxine is critical to the regulation of metabolismMetabolism in the most general sense, is the ingestion and breakdown of complex compounds, coupled with the liberation of energy, and the consequent generation of waste products. It is major process of living organisms, and because this process can happen and growth, throughout the animal kingdom. Among amphibians, for example, administering a thyroid-blocking agent such as propylthiouracil can prevent tadpoles from metamorphosing into frogs; conversely, administering thyroxine will trigger metamorphosis.
In humans, children born with thyroid hormone deficiency will not grow well, and brain development can be severely impaired, in the condition referred to as cretinismCretinism is a congenital form of deficiency of thyroid hormones, retarding mental and physical growth. A cretin is a person affected by cretinism''. This term has come to be regarded as insulting in the general population, although it is appropriate with. Newborn children in many developed countries are now routinely tested for thyroid hormone deficiency; this is done by analysis of a small drop of blood from the child (usually, the blood also is tested for phenylketonuriaPhenylketonuria "fee-nil-kee-ton-yur-ee-aah"+ (PKU) is a human genetic disorder that occurs in about 1 in 15,000 births, but the incidence varies widely in different human populations from 1 in 4,500 births among the Irish to fewer than one in 100,000 bir and several other metabolic diseases of genetic etiology). Children with thyroid hormone deficiency are easily treated by supplementation with synthetic thyroxine, which enables them to grow and develop normally.
Because of the thyroid's selective uptake and extreme concentration of what is actually a quite rare element, it is extremely sensitive to the effects of various radioactive isotopeIsotopes are atoms of a chemical element whose nuclei have the same atomic number, Z but different atomic weights, A''. The word isotope meaning at the same place comes from the fact that isotopes are located at the same place on the periodic table. The as of iodine produced by nuclear fission. In the event of large accidental releases of such material into the environment, the uptake of radioactive iodine by the thyroid can, in theory, be blocked by saturating the uptake mechanism with a large surplus of non-radioactive iodine, taken in the form of iodide tablets. While biological researchers making compounds labelled with iodine isotopes do this, in the wider world such preventive measures are usually not stockpiled before an accident, nor are they distributed adequately afterward - one consequence of the Chernobyl disaster was an increase in thyroid cancers in the years following the accident. [1]