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.
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