Triiodothyronine

Triiodothyronine:

A hormone that helps regulate growth and development, helps control metabolism and body temperature, and, by a negative-feedback system, acts to inhibit the secretion of thyrotropin by the pituitary gland

Structure: 

What is triiodothyronine?

Triiodothyronine is the active form of the thyroid hormone thyroxine.  Approximately 20% of triiodothyronine is secreted into the bloodstream directly by the thyroid gland.  The remaining 80% is produced from conversion of thyroxine by organs such as the liver and kidneys.  Thyroid hormones play vital roles in regulating the body’s metabolic rate, heart and digestive functions, muscle control, brain development and the maintenance of bones.

How is triiodothyronine controlled?

The production and release of thyroid hormones, thyroxine and triiodothyronine, is controlled by a feedback system involving the hypothalamus, pituitary gland and thyroid gland

What happens if I have too much triiodothyronine?

Thyrotoxicosis is the name of the condition in which people have too much thyroid hormone in their bloodstreams. It may result from overactivity of the thyroid gland (hyperthyroidism) from conditions such asGraves' disease, inflammation of the thyroid or a benign tumour.  Thyrotoxicosis may be recognised by agoitre which is a swelling of the neck due to enlargement of the thyroid.  

What happens if I have too little triiodothyronine?

Hypothyroidism is the term for the production of too little thyroid hormone by the thyroid gland.  This may be because of autoimmune diseases, poor iodine intake or the result of taking particular drugs.  Since thyroid hormones are essential for physical and mental development, hypothyroidism before birth and during childhood results in learning disability and reduced physical growth. 

Mechanism of action:

The function of the thyroid gland is to take iodine, found in many foods, and convert it into thyroid hormones: thyroxine (T4) and triiodothyronine (T3).Thyroid cells are the only cells in the body which can absorb iodine. These cells combine iodine and the amino acid tyrosine to make T3 and T4. T3 and T4 are then released into the blood stream and are transported throughout the body where they control metabolism (conversion of oxygen and calories to energy).Every cell in the body depends upon thyroid hormones for regulation of their metabolism. The normal thyroid gland produces about 80% T4 and about 20% T3, however, T3 possesses about four times the hormone "strength" as T4.

The thyroid gland is under the control of the pituitary gland, a small gland the size of a peanut at the base of the brain (shown here in orange). When the level of thyroid hormones (T3 & T4) drops too low, the pituitary gland produces Thyroid Stimulating Hormone (TSH)which stimulates the thyroid gland to produce more hormones. Under the influence of TSH, the thyroid will manufacture and secrete T3 and T4 thereby raising their blood levels. The pituitary senses this and responds by decreasing its TSH production.

The pituitary gland itself is regulated by another gland, known as the hypothalamus (shown in our picture in light blue). The hypothalamus is part of the brain and produces TSH Releasing Hormone (TRH) which tells the pituitary gland to stimulate the thyroid gland (release TSH).