Thyroid Function and Regulation

All of the following statements regarding thyroid hormone synthesis and secretion are correct EXCEPT:

Which of the following best describes the mechanism by which TSH stimulates thyroid hormone secretion?

A 35-year-old woman from Mumbai presents with a 3-month history of progressive fatigue, weight gain of 8 kg despite reduced appetite, and constipation. She reports feeling cold even in warm weather and has noticed her skin becoming dry and coarse. On examination, her heart rate is 52 bpm, blood pressure 128/82 mmHg, and there is a non-tender, diffuse thyroid enlargement. Laboratory investigations show: TSH 18.5 mIU/L (normal 0.4–4.0), free T4 2.1 ng/dL (normal 0.8–1.8), and elevated anti-TPO antibodies (>500 IU/mL). What is the primary mechanism underlying the elevated TSH despite low-normal free T4 in this patient?

A 28-year-old man presents with palpitations, tremor, and weight loss despite good appetite for 2 weeks. Heart rate is 110 bpm, and he appears anxious. Serum TSH is suppressed at 0.05 mIU/L (normal 0.4–4.0). Which investigation is most appropriate to confirm thyroid hormone excess and differentiate the cause?

A 35-year-old woman presents with a 3-month history of fatigue, weight gain of 8 kg, cold intolerance, and dry skin. On examination, she has a non-tender diffuse goitre and delayed relaxation of deep tendon reflexes. TSH is 18 mIU/L (normal 0.4–4 mIU/L) and free T₄ is 8 pmol/L (normal 10–20 pmol/L). Anti-TPO antibodies are positive. What is the most appropriate next step in management?