Thyroid

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Image:Thyroidgland.png
Image:Illu thyroid parathyroid.jpg

In anatomy, the thyroid is the largest endocrine gland in the body. It is situated on the front side of the neck at the level of C5 and T1 vertebral bodies, just below the laryngeal prominence (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 - 15-40 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.

Contents

Histology of the thyroid

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. Twenty-five percent of all the body I- is in the thyroid gland. 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 cells or C cells, which secrete calcitonin.

Causes of thyroid problems

In areas of the world 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 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 phenylketonuria 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 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 potassium 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]

Iodised salt is a very cheap and easy way of adding iodine to the diet.

Diseases and conditions of the thyroid gland

The most common diseases of the thyroid:

The most common conditions of the thyroid:

  • Hypothyroidism
    • Resulting from autoimmune disease
    • Postoperative hypothyroidism
    • Iatrogenic hypothyroidism
  • Hyperthyroidism
    • Resulting from autoimmune disease
    • Iatrogenic hyperthyroidism
  • Other
    • Hashitoxicosis
    • Thyroid Storm
    • Lingual Thyroid
    • Thryoglossal Duct Cyst

Diagnosis

The measurement of thyroid-stimulating hormone (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.

History

The thyroid was first identified by the anatomist Thomas Wharton (whose name is also eponymised in Wharton's duct of the submandibular gland) in 1656. Thyroid hormone (or thyroxin) was only identified in the 19th century.

Blood supply of the thyroid

The thyroid gland is supplied by two arteries: the superior and inferior thyroid arteries. The superior thyroid artery is the first branch of the external carotid, and supplies mostly the upper half of the thyroid gland, while the inferior thyroid artery is the major branch of the thyrocervical trunk, which comes off of the subclavian artery. In 10% of people, there is also a thyroid ima artery that arises from the brachiocephalic trunk or the arch of the aorta.

There are three main veins that drain the thyroid. The superior, middle and inferior thyroid veins.

Surgical Removal of the Thyroid

If the thyroid gland must be removed surgically for any reason, care must be taken to avoid damage to the adjacent structures that are extremely susceptible to accidental removal and/or severence. In particular, the parathyroid glands, which produce parathormone (PTH) reside on the posterior wall of the thyroid gland, and the recurrent laryngeal nerves, which provide motor control for all external muscles of the larynx except for the cricothyroid muscle, also runs along the posterior thyroid. Accidental laceration of either of the two or both recurrent laryngeal nerves will lead to paralysis of the vocal cords and their associated muscles and a subsequent muting effect on the patient.


Endocrine system
Adrenal gland - Corpus luteum - Hypothalamus - Ovaries - Pancreas - Parathyroid gland - Pineal gland - Pituitary gland - Testes - Thyroid gland
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