This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Diseases of Thyroid, Subject name: Medicine
A 40 year old lady presents to OPD with weight loss and palpitations is having hea rate of 110/min, BP=130/70mmHg, bilateral proptosis and warm moist skin. Investigations show undetectable TSH and normal free T4. What is the next best step in diagnosis? 
Select and explain. 
A. RAIU scan, B. TPO antibody screen, C. Thyroid stimulating antibody screen, D. Free T3 levels
D: Free T3 levels
Answer: D: Free T3 levels 
Explanation: The clinical diagnosis is Grave's disease. Since TSH is undetectable it fuher highlights this diagnosis. Since the question says next best step, we first have to demonstrate diagnosis of thyrotoxicosis with values of Free T3. The algorithm shown below highlights the impoance of measurement of free T3 levels. Measurement of thyroid stimulating antibodies is not routinely performed to diagnose Grave's disease.