Toxic multinodular goiter (TMNG) is the second most common cause of hyperthyroidism after Graves disease, resulting from multiple autonomously functioning thyroid nodules that produce thyroid hormone independent of TSH regulation. The pathogenesis involves somatic activating mutations in the TSH receptor (TSHR) gene or the Gs-alpha protein (GNAS1) gene within individual nodules, leading to constitutive activation of the cAMP signaling cascade that drives thyroid hormone synthesis, iodine uptake, and cell proliferation without TSH stimulation. TMNG typically develops over years to decades as a long-standing nontoxic multinodular goiter gradually acquires autonomous function. The nodules progress from polyclonal hyperplastic lesions to monoclonal neoplastic nodules with autonomous function. The excess thyroid hormone production suppresses TSH through negative feedback, which reduces function in the surrounding normal thyroid tissue (visible as photopenic areas on radioiodine scan), while the autonomous nodules continue producing hormones (visible as focal hot areas). TMNG is most common in elderly patients and in regions with historical iodine deficiency. The hyperthyroidism in TMNG is typically milder than in Graves disease, with lower free T4/T3 levels, but elderly patients may present with subclinical hyperthyroidism...