A 58-year-old male presents with palpitations and hand tremors for 3 years, worsening over 1 month. Physical exam: HR 110 bpm, BP 160/60 mmHg, weight loss, moist skin, palpable thyroid gland with vascular bruit, no jugular venous distension, normal cardiac size, HR 134 bpm with irregularly irregular rhythm and varying heart sound intensity, clear lungs and abdomen, no lower limb edema. The most likely etiology is: