THYROID HORMONES

Health Sciences > Medicine > Endocrinology > Thyroid hormones

Thyroid hormones - a class of hormones, synthesised by follicular cells within the thyroid gland. Inside the follicles is found thyroglobulin - a protein that is characterized by high content of the amino acid tyrosine.

Iodide is actively absorbed from the bloodstream and concentrated in the thyroid follicles. (If there is a deficiency of dietary iodine, the thyroid enlarges in an attempt to trap more iodine, resulting in goitre) Via a reaction with the enzyme thyroperoxidase, iodine is covalently bound to tyrosine residues in the thyroglobulin molecules, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT). By linking two moieties of DIT we have thyroxine and by combining one particle of MIT and one particle of DIT we have triiodothyronine.

MIT + DIT = triiodothyronine (usually referred to as T₃) DIT + DIT = thyroxine (referred to as T₄)

The iodinated thyroglobulin is digested by proteases, releasing the hormones - T4 and T3 are the biologically active agents central to regulation of metabolism. Thyroxine is supposedly a prohormone and a reservoir for the most active and main thyroid hormone T₃, T₄ being converted as required in the tissues by deiodinases.

Most of the thyroid hormone circulating in the blood is bound to transport proteins :

• thyroid binding globulin (TBG)
• Thyroid Binding Prealbumin (TBPA) - this protein is also responsible for the transport of retinol, and so now has the preferred name of transthyretin (TTR)
• albumins

Only a very small fraction of the circulating hormone is free (unbound) - T₄ 0.03% and T₃ 0.3%. This free fraction is biologically active, hence measuring concentrations of free thyroid hormones is of great diagnostic value.
These values are referred to as fT₄ and fT₃.

There are numerous physiological and pathological stimuli that influence the synthesis of thyroid hormones.

The hormones are essential to proper development and differentiation of all cells of human body. To various extent they regulate protein, fat and carbohydrate metabolism. The most pronounced impact, however, is on utilization of energetic compounds by human cellss.

Thyrotoxicosis or hyperthyroidism is the clinical syndrome caused by an excess of circulating free thyroxine and free triiodothyronine, or both. It is a common disorder and affects approximately 2% of women and 0.2% of men.

See also:

• Endocrine system