Thyroid gland

(Date of publication 21 May 2004)

This gland in the front of the neck plays an important role in regulating the metabolism of nearly all the tissues of the body. In health it is small and butterfly-shaped, consisting of right and left lobes connected across the midline by a narrow isthmus, but the magnitude of pathological enlargement can be spectacular. The anatomy of the thyroid and its associated parathyroid glands is outlined here, but the descriptions here are very brief - this site is primarily worth visiting for the photographs and microscope slides, which clearly illustrate the thyroid's distinctive microscopic structure. Epithelial cells, responsible for synthesising thyroid hormones, are arranged in thyroid follicles filled with colloid, the hormone precursor thyroglobulin. In the spaces between follicles are parafollicular cells that secrete calcitonin, which lowers serum levels of calcium and phosphate. For a clear image at 10´ magnification, try here.

To discover more about the gross anatomy of the gland and its related structures, what could be more authoritative than Gray's Anatomy, even if it is the 1916 edition? However, you may prefer a more systematic approach which deals separately with, for example, the thyroid's vascular anatomy, venous drainage and innervation. If so, go to the emedicine site, to which we will be returning later in the tour.

The two principal thyroid hormones are thyroxine (T4) and triiodotyronine (T3), both derivatives of the amino acid tyrosine. As can be seen from the structural formulae, they essentially consist of two linked tyrosine molecules with the addition of iodine at three or four points on the aromatic rings. Much more thyroxin is secreted than the considerably more active triiodotyronine; the bulk of T3 (80%) is obtained by de-iodination of thyroxin in peripheral tissues, especially the liver and kidney. The mechanism by which these hormones are synthesised and released is described in the Merck Manual. Once in the tissues, they have two major physiological effects; they increase protein synthesis, and T3 raises oxygen consumption by increasing the activity of the sodium pump, primarily in tissues such as the liver, kidney, heart and skeletal muscle.

Common thyroid problems are classified on the EndocrineWeb.com website according to whether they involve a change in hormone production, compression resulting from increased growth, the development of lumps or nodules, or neoplasia. Each topic heading is linked to a lengthier explanation which includes patient X-rays and photographs.

Hypothyroidism is a failure to maintain adequate tissue levels of thyroid hormone. It may be primary, secondary, or tertiary. Hypothyroidism in adults, or myxoedema, has many causes and gives rise to symptoms such as fatigue, lethargy, muscle and joint pain, depression, cold intolerance and an inability to concentrate. There is also a characteristic dry, firm, waxy swelling of the skin and subcutaneous tissues - see the photograph here. Treatment consists of replacing the deficient hormone with synthetic T3, T4 or desiccated thyroid extracts, the objective being to achieve a clinical response and normal blood levels of TSH and T4.

The early diagnosis of congenital hypothyroidism is vital, because critical development of the nervous system takes place in the first few months after birth; if left untreated, irreversible damage and marked mental impairment may result. Incomplete development of the thyroid is the most common cause of the condition, which frequently gives severely-affected infants a characteristic appearance with a puffy face, large protruding tongue and open mouth. Fortunately, most effects of hypothyroidism are easily reversed and hormone replacement therapy generally results in a good outcome.

The term goitre is non-specific and refers only to enlargement of the thyroid gland; it does not imply the presence of any specific pathology. The two types of goitre, diffuse and multinodular, may be further subdivided and this classification is useful when looking at hyperthyroidism, also termed thyrotoxicosis.

The over-production of thyroid hormones in hyperthyroidism increases the metabolic rate, producing agitation, restlessness, rapid heart rate, feelings of excessive warmth, weight loss, severe tiredness and muscle pains. There are three main types of hyperthyroidism. Diffuse toxic goitre, or Grave's Disease, is the most common and affects mainly young people. The whole gland is swollen, with most sufferers experiencing some sort of eye problem, ranging from minor irritation to marked exophthalmos as in the photograph here. By contrast, toxic nodular goitre afflicts mainly the elderly; the thyroid is often non-uniform in shape and rough, and eye conditions are not usually a feature. The third type is subacute or De Quervain's thyroiditis, an inflammation probably resulting from viral infection that leads to the release of stored hormones. A description of other, less frequently encountered, causes and much additional information can be found here. Three types of therapy are available: drugs such as carbimazole and propylthiouracil reduce the production of T3 and T4, subtotal thyroidectomy involves removing most of the gland surgically, and radioactive iodine taken orally usually returns the metabolism to normal within months or even weeks.

Of the three main types of thyroiditis, or inflammation of the thyroid, by far the most common is Hashimoto's Disease, which runs in families and affects about 5% of adults in the USA. Essentially, it is an autoimmune phenomenon in which thyroid tissue is attacked and destroyed - look at the abnormal appearance of the gland on this photomicrograph. The process is very slow, so that some cases remain stable for years, while others gradually progress to hypothyroidism, which can be treated with hormone replacement therapy.

Finally, thyroid cancers are rare, but almost always curable. The surgical treatment of the four major types - papillary, follicular, medullary and anaplastic - is described on this US ENT site. Evidence is cited that total thyroidectomy significantly decreases the recurrence rate and the number of deaths when compared with subtotal removal, and this approach is recommended for all except anaplastic tumours, which are rarely resectable. It has now become recognised that previous exposure to irradiation of the head and neck is an important contributory factor in the development of thyroid cancer, yet years ago X-rays were administered for acne, tonsillitis - even excess facial hair! Ah, the benefits of hindsight...

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A Tour around the Thyroid Gland

A Tour around the Thyroid Gland