A 45-year-old woman presents with fatigue, weight gain, constipation, and dry skin. On examination, she has delayed deep tendon reflexes and periorbital puffiness. Blood tests show: TSH: 15 mIU/L (high) Free T4: 8 pmol/L (low) What is the most likely cause? A) Hashimoto’s thyroiditis B) Graves’ disease C) Pituitary adenoma D) Subacute thyroiditis E) Iodine-induced hypothyroidism Correct Answer: A) Hashimoto’s thyroiditis Explanation: Primary hypothyroidism: ↑ TSH, ↓ Free T4 Most common cause = Hashimoto’s thyroiditis (autoimmune) Symptoms: Fatigue, weight gain, dry skin, cold intolerance, delayed reflexes Diagnosis: Anti-TPO (thyroid peroxidase) antibodies B) Graves’ disease → Incorrect. Causes hyperthyroidism (weight loss, tremors, exophthalmos). C) Pituitary adenoma → Incorrect. Would cause low TSH and low T4 (secondary hypothyroidism). D) Subacute thyroiditis → Incorrect. Usually presents with painful thyroid + transient hyperthyroidism. E) Iodine-induced hypothyroidism → Incorrect. Rare; seen in patients with underlying thyroid disease. Key Tips for PLAB 1: ✅ Primary hypothyroidism = ↑ TSH + ↓ T4. ✅ Autoimmune cause: Check Anti-TPO antibodies. ✅ Treatment: Levothyroxine (adjust dose based on TSH levels).
