How does the thyroid affect vital organ function?
The thyroid gland secretes two active hormones: triiodothyronine (T3) and thyroxine (T4). These hormones increase the basal metabolic rate through unclear mecha-nisms and results in an increase in oxygen consumption and heat generation.
Thyroid hormones increase cardiac output by increasing heart rate, preload, and contractility. These changes occur because of the increase in oxygen demand. Excessive levels of thyroid hormones result in up-regulation of β-adrenergic receptors in all tissues. This change in β-receptors results in an exaggerated response to sympathetic stimulation and to the administration of exogenous catecholamines such as ephedrine or epinephrine. Decreased levels of thyroid hormones have the opposite effect, and can make the indi-vidual unresponsive to even large doses of β-agonists.
Hyperthyroidism can lead to angina, high-output cardiac failure, and atrial fibrillation. Hypothyroidism can lead to cardiomegaly and decreased cardiac output.