Thyroid

The thyroid is an endocrine gland. It is situated on the front side of the neck, 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 - and butterfly-shaped: the wings correspond to the lobes and the body to the isthmus of the thyroid.

The primary function of the thyroid is production of hormones:

• thyroxine (T4)
• triiodothyronine[?] (T3)
• and calcitonin[?], which regulates calcium-phosphorus metabolism

The gland is composed of follicular tissue that selectively absorbs iodine (more accurately iodide ions, I^-) from the bloodstream and concentrates it for production of thyroid hormones. The follicle is built of a single layer of epithelial cells, surrounding a colloid rich in a protein called thyreoglobuline[?]. It serves as a reservoir of materials for thyroid hormone production and, to a lesser extent, a reservoir of the hormones themselves.

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 goitre.

Thyroxine is critical to the regulation of metabolism 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 cretinism. Newborn children in many developed counties 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 is also tested for phenylketonuria at the same time). 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 isotopes 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. Biological researchers making compounds labelled with iodine isotopes do this, but in the wider world such countermeasures are not necessarily stockpiled before an accident, or distributed adequately after - one consequence of the Chernobyl disaster was an increase in thyroid cancers in the years following the accident. (See http://news.bbc.co.uk/hi/english/sci/tech/newsid_1319000/1319386.stm)

The most common diseases of the thyroid:

• Hypothyroidism
  • postoperative hypothyroidism
  • iatrogenic hypothyroidism
• Hyperthyroidism
  • Graves disease[?]
  • toxic thyroid nodule
• Thyroid nodules[?]
• Thyroid cancer
• Goitre
  • endemic goitre
  • diffuse goitre[?]
  • multinodular goitre[?]
• Thyroiditis[?]
  • Hashimoto's thyroiditis[?]
  • Acute thyroiditis[?]

Diagnosis

The measurement of TSH[?] levels is useful in the diagnosis of hypothyroidism (thyroid hormone deficiency), since TSH levels are often elevated before decreased levels of thyroid hormones T4 and T3 are detectable.