Hyperthyroidism II: Pathophysiology

Graves' disease is an autoimmune disorder where thyroid-stimulating antibodies activate TSH receptors on thyroid follicular cells, mimicking TSH action. This drives continuous thyroid hormone production, bypassing the normal negative feedback mechanism that typically regulates the hypothalamic-pituitary-thyroid axis.

Elevated T3 and T4 increase basal metabolic rate by enhancing mitochondrial activity and oxygen consumption. They also upregulate beta-adrenergic receptors, making tissues more responsive to catecholamines and causing symptoms like palpitations, tremors, and heat intolerance.

Toxic multinodular goiter and toxic adenoma produce excess thyroid hormone independently of TSH regulation. Pituitary adenomas that secrete excess TSH can also stimulate the thyroid gland despite already elevated hormone levels, causing hyperthyroidism through a different mechanism.

Elevated thyroid hormones accelerate bone turnover, reducing bone mineral density and increasing osteoporosis risk. Muscle protein breakdown contributes to weakness. These metabolic changes can have serious long-term consequences if hyperthyroidism remains untreated.

Increased gastrointestinal motility leads to more frequent stools. Thyroid hormones stimulate hepatic glucose production and accelerate insulin clearance, potentially impairing glucose tolerance or worsening existing diabetes. Lipid levels also decline significantly.

Untreated hyperthyroidism may cause atrial fibrillation, heart failure, osteoporosis, and fertility issues. In Graves' disease, immune-mediated inflammation can result in Graves' ophthalmopathy and pretibial myxedema. Thyroid storm, a life-threatening emergency with fever and multi-organ dysfunction, can occur in severe cases.

Disruptions in reproductive hormone balance lead to menstrual irregularities and infertility in hyperthyroidism. These hormonal imbalances result from the systemic effects of elevated T3 and T4 on the endocrine system, affecting fertility and sexual function.