Hyperthyroidism II: Pathophysiology

Hyperthyroidism is characterized by an overproduction of the thyroid hormones triiodothyronine, or T3, and thyroxine, or T4, by the thyroid gland, leading to elevated circulating hormone levels.

The most prevalent cause is Graves’ disease, an autoimmune disorder in which thyroid-stimulating antibodies activate thyroid-stimulating hormone or TSH receptors on thyroid follicular cells.

This mimics the action of TSH, leading to uncontrolled hormone production. In turn, high levels of T3 and T4 suppress TSH release through negative feedback on the hypothalamic-pituitary-thyroid axis.

Less common causes include toxic multinodular goiter and toxic adenoma, which produce excess thyroid hormone independently of TSH, and pituitary adenomas that secrete excess TSH.

Elevated thyroid hormones increase mitochondrial activity, oxygen consumption, and basal metabolic rate.

They also enhance beta-adrenergic receptor expression, heightening sensitivity to catecholamines and causing symptoms like palpitations, tremors, anxiety, and heat intolerance.