A 28-year-old woman presents with a 2-week history of anterior neck and throat pain, accompanied by heat intolerance, palpitations, hyperhidrosis, and generalized fatigue. Physical examination reveals temperature 38°C, weight loss, and moist skin. No exophthalmos. Thyroid gland is grade I enlarged with bilateral tenderness. No bruit. Fine tremor in both hands (+). What is the most likely initial diagnosis?