A 45-year-old woman presents with progressive fatigue, constipation, and 8-pound weight gain over three months. Vital signs show BP 128/82, HR 58, RR 14, temp 36.8°C, SpO2 98%. Physical examination reveals a diffusely enlarged, painless thyroid. Laboratory studies demonstrate TSH 12.4 mIU/L (normal 0.4–4.0), free T4 0.6 ng/dL (normal 0.8–1.8), and positive anti-thyroid peroxidase antibodies. Thyroid ultrasound shows homogeneous echotexture without nodules. She denies neck pain. Which of the following is the most likely diagnosis?

1. A)Hashimoto thyroiditisGABARITO
2. B)Toxic multinodular goiter
3. C)Medullary thyroid carcinoma
4. D)Graves disease
5. E)Subacute granulomatous thyroiditis