Thyroid Hormone Synthesis and Secretion

A 35-year-old man from Bangalore presents with a 2-month history of palpitations, tremor, heat intolerance, and weight loss despite good appetite. On examination, he has a warm, moist skin, resting heart rate of 110 bpm, and a non-tender, diffuse thyroid enlargement with an audible bruit. Serum TSH is <0.1 mIU/L (suppressed), free T4 is 28 pmol/L (elevated), free T3 is 9.2 pmol/L (elevated), and TSI (thyroid-stimulating immunoglobulin) is positive. Which of the following mechanisms best explains the suppressed TSH in the presence of elevated thyroid hormones?

A 28-year-old man with a known history of Hashimoto's thyroiditis is being evaluated for persistent symptoms despite levothyroxine replacement. His TSH is 2.1 mIU/L (normal: 0.4–4.0) and FT4 is 12 pg/mL (normal: 9–17). He complains of fatigue and poor concentration. Which investigation is most appropriate to assess his thyroid hormone metabolism and peripheral conversion?

A 28-year-old woman presents with thyrotoxicosis at 10 weeks of pregnancy. She is started on an antithyroid drug that inhibits the enzyme marked **B** in the diagram, blocking iodide oxidation, organification, and coupling of iodinated tyrosine residues. Which of the following is the most appropriate choice for this patient at this gestational age?

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

A 28-year-old man with a history of Graves disease treated with antithyroid drugs 2 years ago now presents with palpitations, tremor, and heat intolerance. TSH is suppressed at 0.1 mIU/L (normal 0.4–4.0), and free T4 is elevated at 22 ng/dL (normal 7–18). Which investigation is most specific for determining whether he has relapsed Graves disease or developed thyroiditis?