MRCP2-1554

MRCP2-1554

A 42-year-old woman presents with a recent history of weight loss and anxiety. She had been diagnosed with thyrotoxicosis three years ago and had been treated with carbimazole, but failed to attend follow-up appointments for over a year. Although she felt better after the treatment, she still had a goitre. Recently, she noticed a more prominent swelling on the right side of her neck and experienced symptoms of anxiety, along with a 3 kg weight loss. She is currently not taking any medication but is a smoker of 10 cigarettes daily.

During the examination, the patient had a pulse of 96 beats per minute, a fine tremor of the outstretched hands, lid lag, and some periorbital puffiness. There was a moderately enlarged and diffuse goitre with a more prominent 3 cm nodule on the left of the gland, which was non-tender. A bruit was heard over the goitre, and no lymphadenopathy was palpable. No other abnormalities were noted.

The investigations revealed a free T4 level of 37.3 pmol/L (10-22), a TSH level of 0.05 mU/L (0.4-5), and thyroid peroxidase antibodies of 1:2400 U/L. The I123 uptake scan showed diffuse uptake with no uptake in the left nodule. What is the most likely cause of the thyroid nodule?