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Hyperthyroidism is a hypermetabolic state caused by elevated levels of thyroid hormones, triiodothyronine (T3) and thyroxine (T4). It results from dysregulation at the thyroid, pituitary, or immune system level and affects multiple organ systems.
Pathophysiology
The most common cause of hyperthyroidism is Graves’ disease, an autoimmune disorder in which antibodies, specifically thyroid-stimulating antibodies (TSAb), a subtype of TSH receptor antibodies (TRAb), bind to and activate TSH receptors on thyroid follicular cells. This drives continuous thyroid hormone production, bypassing the usual negative feedback mechanism.
Other causes include toxic multinodular goiter and toxic adenoma, where thyroid nodules function autonomously, producing excess hormone without TSH regulation. A pituitary adenoma may secrete excess TSH, stimulating the thyroid gland despite already elevated hormone levels.
Functional Effect of Hyperthyroidism
Elevated thyroid hormone levels increase basal metabolic rate by enhancing mitochondrial activity and oxygen consumption. They also upregulate β-adrenergic receptors, making tissues more responsive to catecholamines—resulting in symptoms such as palpitations, heat intolerance, tremors, and anxiety.
Muscle protein breakdown contributes to weakness, and accelerated bone turnover can reduce bone mineral density. Gastrointestinal motility increases, leading to more frequent stools. Thyroid hormones also stimulate hepatic glucose production and insulin clearance, which may impair glucose tolerance or exacerbate existing diabetes. Lipid levels decline, and disruptions in reproductive hormone balance can lead to menstrual irregularities and infertility.
Clinical Manifestations
If left untreated, hyperthyroidism may lead to serious complications such as atrial fibrillation, heart failure, osteoporosis, and fertility issues. In Graves’ disease, immune-mediated inflammation can result in Graves’ ophthalmopathy, which affects the eyes, and pretibial myxedema, which affects the skin. In rare but severe cases, extreme hormone excess can trigger a thyroid storm—a life-threatening emergency characterized by fever, tachycardia, and multi-organ dysfunction.
