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 function, and the maintenance of bones.
The production and release of thyroid hormones, thyroxine and triiodothyronine, is controlled by a feedback loop involving the hypothalamus, pituitary gland and thyroid gland. Activation of thyroid hormones is then controlled in body tissues such as the liver, brain and kidneys by enzymes called deiodinases which convert thyroxine into the active form triiodothyronine. Most of the body’s circulating triiodothyronine (about 80%) is produced in this way.
The thyroid hormone production system is regulated by a feedback loop so that when the levels of the thyroid hormones thyroxine and triiodothyronine increase, they prevent the release of both thyrotropin-releasing hormone from the hypothalamus and thyroid stimulating hormone from the pituitary gland. This system allows the body to maintain a constant level of thyroid hormones in the body.
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 as Graves' disease, inflammation of the thyroid or a benign tumour. Thyrotoxicosis may be recognised by a goitre, which is a swelling of the neck due to enlargement of the thyroid. Other symptoms of thyrotoxicosis include heat intolerance, weight loss, increased appetite, increased bowel movements, irregular menstrual cycle, rapid or irregular heartbeat, palpitations, tiredness, irritability, tremor, hair thinning/loss and retraction of the eyelids, which results in a ‘staring’ appearance.
Hypothyroidism is the term for the production of too little thyroid hormone by the thyroid gland. This may be because of autoimmune diseases (such as Hashimoto’s disease), very poor iodine intake or due to some medications. Since thyroid hormones are essential for physical and mental development, untreated hypothyroidism before birth and during childhood can result in learning disability and reduced growth.
Hypothyroidism in adults results in a slowing of the body’s functions with symptoms such as tiredness, intolerance to cold temperatures, low heart rate, weight gain, reduced appetite, poor memory, depression, stiffness of the muscles and reduced fertility. See the article on hypothyroidism for more information.
Last reviewed: Mar 2018