Is there a role for 18F-FDG PET/CT in the diagnosis of painless sub-acute thyroiditis?

H Esdaile, K Narula, S Qureshi, M Martineau, R Kaushal, West Middlesex Hospital, London.

Abstract: Whilst focal and diffuse FDG uptake in the thyroid gland is one of the incidental findings encountered during the routine clinical use of 18F-FDG PET/CT, there are few cases where 18F-FDG PET/CT scans have been performed on patients with subacute thyroiditis (SAT).

We present a lady treated for SAT who had incidentally had a surveillance 18F-FDG PET/CT for breast cancer just before the onset of her symptoms.

A 55 year old lady presented to the emergency department with a 5 week history of palpitations, fever and anxiety after finishing a two week course of antibiotics. Her Covid19 screen was negative and she denied any neck swelling or pain. No infective cause was identified. She was taking Exemestane and Zometa for her HER2 receptor positive breast cancer with metastases in her spine. She had not received immunotherapy in the past. Her routine surveillance 18F-FDG PET/CT 6 weeks previously described bilateral focal thyroid FDG uptake, and the appearances of an ultrasound organised in primary care a week previous to the presentation described diffuse changes in the thyroid in keeping with a likely post viral granulomatous thyroiditis. There were no visible nodules or any suspicious radiological features on imaging. She was febrile (39.1 degrees centigrade) and tachycardic on admission with an elevated free T4 46.8 p mol/l, free T3 11.6 p mol/L and a suppressed TSH < 0.01 m IU/L. Her CRP and WCC count were both elevated and her TSH receptor antibodies were negative. She was initiated on Prednisolone 20 mg orally daily, in addition to Propranolol for SAT. Her symptoms resolved within 48 hours and she was discharged. After two weeks her T4 had improved to 18.7 p mol/L and free T3 to 5.6 pmol/L. Her oral steroids were subsequently weaned off.

Painful SAT associated with De Quervain or granulomatous thyroiditis is normally characterised with a painful swelling in the thyroid and resolves spontaneously. Painless SAT is less frequent and is mainly associated with post-partum, Hashimoto’s, or drug associated thyroiditis. 18F-FDG PET/CT has an established role in oncology imaging but may be useful to ascertain inflammatory activity in painless SAT. In this latter group of patients, case reports have observed that appearances are generally of diffuse, occasional focal, asymmetrical uptake in the thyroid on 18F-FDG PET/CT, even in patients who have concurrently undergone a Tc-99m pertechnetate scan showing no uptake. Our case illustrates utilization of 18F-FDG PET/CT for differentiating between different forms of painless SAT associated with thyrotoxicosis, which can be difficult to establish with standard diagnostic radiological procedures.