T4; tetraiodothyronine; thyroxin. The manufactured form used for thyroid hormone replacement is called L-thyroxine or Levothyroxine.
Thyroxine is the main hormone secreted into the bloodstream by the thyroid gland. It is the less active form of thyroid hormone and most of it is converted to a more active form called triiodothyronine (T3) by local tissues including the liver, kidneys, skeletal and heart muscles, central nervous system, skin, etc. Thyroid hormones play vital roles in regulating the body’s metabolic rate, heart, digestive function, muscle control, brain development and maintenance of bones.
The production and release of thyroid hormones, thyroxine (T4) and triiodothyronine (T3), 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 (TSH). This hormone stimulates the production of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3), by the thyroid gland.
This hormone production system is regulated by a feedback loop so that when the levels of the thyroid hormones (thyroxine and triiodothyronine) are too high, they exert negative feedback such that the hypothalamus releases less thyrotropin-releasing hormone (TRH) and the pituitary gland releases less thyroid stimulating hormone (TSH). This system allows the body to maintain a constant level of thyroid hormones in the body.
The presence of too much thyroxine in the bloodstream is known as thyrotoxicosis. This may be caused by excessive intake of exogenous thyroid hormones, inflammation of the thyroid gland with consequent release of excess hormones in the bloodstream, or overactivity of the thyroid gland (hyperthyroidism), as in Graves' disease (autoimmune disease) or a benign tumour (Plummer’s adenoma or toxic nodular goitre). Main symptoms of thyrotoxicosis include intolerance to heat, excess sweating, weight loss, increased appetite, increased bowel movements, irregular menstrual cycle, rapid or irregular heartbeat, palpitations, tiredness, irritability, tremor, hair thinning/loss.
The commonest cause of thyrotoxicosis is Graves’ diseasewhich can also have specific signs, especially affecting the eyes, causing them to feel dry / itchy, or appear protruded, or have retraction of the eyelids resulting in a ‘staring’ appearance. Depending on the cause underlying thyrotoxicosis, a goitre can also be present, which is a swelling of the neck due to enlargement of the thyroid gland.
Too little production of thyroxine by the thyroid gland is known as hypothyroidism. It may be caused by autoimmune diseases, poor iodine intake, chemicals (known as endocrine disruptors), or by the use of certain medications. Sometimes, the cause is unknown. Thyroid hormones are essential for physical and mental development so untreated hypothyroidism before birth or during childhood can cause mental impairment and reduced growth.
Hypothyroidism in adults causes reduced metabolism. It can result in symptoms such as fatigue, intolerance of cold temperatures, low heart rate, weight gain, reduced appetite, poor memory, depression, stiffness of the muscles and reduced fertility.
Last reviewed: Jul 2021