How to diagnose amiodarone-induced thyrotoxicosis?

Mr BK, a 55-year-old male has presented with chief complaints of generalized weakness of 2 months duration, weight loss of two months duration and intermittent palpitation for last 15 days. He is a known case of hypertension on treatment. He had an acute anterior wall myocardial infarction one year ago for which he had to undergo percutaneous transluminal coronary angioplasty. Postcoronary intervention patient developed ventricular tachycardia on three occasions (one in ICU and two times in the ward). For which, he was electroverted and was put on amiodarone (200 mg tablets three times a day since then). Current clinical examination reveals sinus tachycardia with heart rate of 110 per minute, blood pressure is 126/80 mm Hg. He has a grade-1 soft diffuse goiter. On evaluation, his routine hemogram with renal function and liver function tests are normal. His thyroid function test reveals serum T3 of 190 ng/dL (normal range, 80–200 ng/dL), serum T4 of 22 mg/dL (normal range 5.1–14.1 mg/dL) and serum TSH of 0.01 mIU/mL (normal range, 0.27–4.20 mIU/ mL). Serum anti-TPO antibody is within normal range.

How to diagnose amiodarone-induced thyrotoxicosis?