Thyroid hormone receptor
Also known as: TR, THRA, THRB, nuclear thyroid receptor
The nuclear receptor in nearly every cell through which triiodothyronine (T3) sets the pace of metabolism, heart rate, temperature and growth. Levothyroxine replaces the thyroid's output and acts on it after conversion to T3.
- Kind
- receptor
- Also called
- TR, THRA, THRB, nuclear thyroid receptor
- Where it is found
- Nuclei of cells in virtually all tissues, with distinct receptor subtypes dominant in heart, brain, liver and pituitary.
What it does
The thyroid secretes mostly thyroxine (T4), which tissues convert to the active hormone T3 with deiodinase enzymes; T3 enters the nucleus and binds thyroid hormone receptors that switch genes on or off. In the heart the result is a faster rate and stronger contraction, in metabolism a higher resting energy expenditure and heat production, in the liver lower cholesterol, in bone higher turnover, and in the developing brain normal maturation, which is why newborns are screened for hypothyroidism. Levothyroxine is synthetic T4: taken once daily on an empty stomach it is converted to T3 as the body requires, giving steady levels with a half-life of about a week. Dosing is judged by the TSH, which the pituitary lowers as replacement becomes adequate. Too much replacement drives the same receptors excessively and causes palpitations, atrial fibrillation, weight loss, anxiety and bone loss; too little leaves the tiredness, weight gain, cold intolerance and slowed heart of hypothyroidism. Iron, calcium, antacids and proton pump inhibitors cut absorption of the tablet.
Role in the body
Transcriptional control of metabolic rate, cardiac output, thermogenesis, lipid metabolism, growth and neurodevelopment.