Endocrine Abstracts

Radiation to the neck is known to be associated with the later development of hypothyroidism. The possibility of acute radiation-induced thyrotoxicosis is not generally recognised. We report here a case of acute hyperthyroidism shortly after radiotherapy.

A 57-year-old man with poorly differentiated adenocarcinoma of the left parotid underwent parotidectomy with radical neck dissection followed by radiotherapy. He received 65 Gy by intensity modulated radiotherapy in 30 fractions. 16 days after completing radiotherapy he developed a sore neck and palpitations. Thyroid function test showed TSH 0.02 mU/l, fT₄ 30.6 pmol/l, fT₃ 8.2 pmol/l, indicating thyrotoxicosis. He was treated with carbimazole and propranolol by his GP, and referred for specialist care. There was no past or family history of thyroid disease, and there were no symptoms or signs to suggest Graves’ disease. TPO was negative. After 11 days of treatment the fT₄ had fallen to 23.9 pmol/l, and fT₃ was normal at 5.8 pmol/l. Radiation-induced thyroiditis was suspected, and after 18 days of treatment carbimazole was stopped. 27 days after diagnosis a ^99mTc thyroid uptake scan showed almost no uptake, indicating acute thyroiditis. After 11 weeks thyroid function returned to normal (TSH 3.38 mU/l); 4 weeks later he developed hypothyroidism (TSH 9.18 mU/l, fT₄ 11.5 pmol/l, TSH later 17.84 mU/l) and was treated with thyroxine.

Therapeutic radiation to the neck is known to increase the risk of thyroid dysfunction, with late hypothyroidism the most common abnormality (20–30% incidence with long-term follow-up). Thyrotoxicosis is also reported, but mostly in people with Hodgkin’s disease, where there is a 7–20-fold higher risk of developing Graves’ thyrotoxicosis after radiotherapy. Thyrotoxicosis due to radiation-induced thyroiditis is uncommon, with only isolated case reports in the literature. These mostly mention asymptomatic thyroiditis; clinically evident thyrotoxicosis is not generally recognised. In thyrotoxicosis due to acute thyroiditis, thionamide therapy should be avoided. Although screening such patients for late hypothyroidism is widely advocated, the value of assessing for hyperthyroidism early after radiotherapy is not known. Thyrotoxicosis from acute thyroiditis after neck irradiation may occur more commonly than is recognised; a prospective study would clarify this.