Expansion
Increases basal metabolic rate and potentiates catecholamine effects
Expansion
Metabolic
- Increases basal metabolic rate and heat production
- Increases glucose absorption, gluconeogenesis and lipolysis
- Increases both synthesis and degradation of protein, with net catabolism in excess
Cardiovascular
- Increases beta-adrenergic receptor density, so cardiac output, heart rate and contractility rise. Catecholamine levels are normal or low, which is why beta blockers relieve symptoms so effectively
Growth and development
- Essential for normal brain development in fetal and neonatal life; deficiency causes irreversible intellectual disability (cretinism), which is why newborn screening exists
- Potentiates growth hormone and promotes bone maturation
Other: increases gut motility, bone turnover, and the speed of reflexes.
The clinical pictures follow directly. Excess: weight loss despite appetite, heat intolerance, tremor, tachycardia, diarrhoea, brisk reflexes. Deficiency: weight gain, cold intolerance, bradycardia, constipation, slow-relaxing reflexes, and in children growth failure.