Thyroid Hormone Functions and Regulation

Metabolic Functions

T3 regulates daily rhythms in glucose metabolism, with faster glucose clearance occurring during the early active phase 1
T3 controls lipolysis and fatty acid oxidation during fasting states, influencing serum triglyceride levels and overall lipid metabolism 1

T3 increases the force and speed of systolic contraction and the speed of diastolic relaxation in cardiac tissue 1, 2
T3 decreases systemic vascular resistance, including coronary vascular tone 1, 2
T3 increases resting heart rate and left ventricular contractility 1, 2
T3 enhances isovolumic ventricular relaxation, improving cardiac function 1
T3 increases coronary arteriolar angiogenesis 2
The American Heart Association acknowledges that the hemodynamic effects of thyroid hormones include decreased systemic vascular resistance, increased resting heart rate, increased LV contractility, and enhanced isovolumic ventricular relaxation 2

T3 and T4 production is primarily regulated by thyroid-stimulating hormone (TSH) from the pituitary gland 1, 3
TSH controls multiple aspects of thyroid hormone synthesis, including stimulation of iodide uptake, activation of thyroid peroxidase enzyme, and regulation of T4 and T3 secretion 3
T3 participates in TSH secretion regulation through negative feedback 1

T3 is crucial for bone health, influencing the production of collagen type I through hydroxylases 1
T3 affects bone mineral density and overall bone health 1
T3 promotes both physiological and pathological myocardial hypertrophy 2
T3 can decrease pulmonary vascular resistance 2
T3 shows distinct daily rhythmic patterns that differ between diurnal humans and nocturnal rodents 1

Abnormal T3 and T4 levels can lead to thyroid dysfunction, manifesting as either hypothyroidism (insufficient thyroid hormones) or hyperthyroidism (excess thyroid hormones) 1, 4
T3 deficiency can result in decreased metabolic rate, while excess T3 increases metabolic rate 1, 3
Iron deficiency can impair the conversion of T4 to T3, affecting thyroid function 1, 3
Selenium is required for the deiodination of T4 to T3 via the selenoenzyme Type 1 5'-deiodinase 1, 3
The American College of Cardiology notes that hypothyroidism promotes myocardial fibrosis by stimulating fibroblasts, whereas hyperthyroidism has the opposite effect 2