T4; tetraiodothyronine; thyroxin
Thyroxine is the main hormone secreted into the bloodstream by the thyroid gland. It is the inactive form and most of it is converted to an active form called triiodothyronine 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 maintenance of bones.
The production and release of thyroid hormones, thyroxine and triiodothyronine, is controlled by a feedback loop system that involves the hypothalamus in the brain and the pituitary and thyroid glands. The hypothalamus secretes thyrotropin-releasing hormone which, in turn, stimulates the pituitary gland to produce thyroid stimulating hormone. This hormone stimulates the production of the thyroid hormones, thyroxine and triiodothyronine, by the thyroid gland.
This hormone production system is regulated by a negative feedback loop so that when the levels of the thyroid hormones, thyroxine and triiodothyronine increase, they prevent the release of both thyrotropin-releasing hormone and thyroid stimulating hormone. This system allows the body to maintain a constant level of thyroid hormones in the body.
The release of too much thyroxine in the bloodstream is known as thyrotoxicosis. This may be caused by overactivity of the thyroid gland (hyperthyroidism), as in Graves' disease, inflammation of the thyroid or a benign tumour. Thyrotoxicosis can be recognised by a goitre, which is a swelling of the neck due to enlargement of the thyroid gland. Other symptoms of thyrotoxicosis include intolerance to heat, weight loss, increased appetite, increased bowel movements, irregular menstrual cycle, rapid or irregular heartbeat, palpitations, tiredness, irritability, tremor, hair loss and retraction of the eyelids resulting in a ‘staring’ appearance.
Too little production of thyroxine by the thyroid gland is known as hypothyroidism. It may be caused by autoimmune diseases, poor iodine intake or brought on by the use of certain drugs. Sometimes, the cause is unknown. Thyroid hormones are essential for physical and mental development so hypothyroidism during development or before birth and during childhood causes mental impairment and reduced physical growth.
Hypothyroidism in adults causes a decreased metabolic rate. This results in symptoms that include fatigue, intolerance of cold temperatures, low heart rate, weight gain, reduced appetite, poor memory, depression, stiffness of the muscles and infertility.
Last reviewed: Jan 2015