An intrathyroidal cystic parathyroid adenoma localised with [¹⁸F]fluorocholine PET/CT in pregnancy

Primary hyperparathyroidism during pregnancy is rare but associated with substantial maternal and foetal morbidity, necessitating timely diagnosis and intervention. Intrathyroidal parathyroid adenomas account for as few as 0.7% of parathyroid adenomas, while cystic parathyroid adenomas represent an even smaller subset. The coexistence of both is exceedingly uncommon, with only nine cases reported and none previously described in pregnancy or using [¹⁸F]fluorocholine PET/CT for localisation. We describe a 37-year-old pregnant woman with PTH-dependent hypercalcaemia which persisted following initial parathyroidectomy. A left thyroid lesion with mixed cystic and peripheral enhancing components was detected on ultrasound and multiphase IV contrast CT (4D CT) but remained indeterminate. To minimise foetal radiation exposure, an ultra-low dose protocol [¹⁸F]fluorocholine PET/CT was performed at 16 weeks' gestation, demonstrating peripheral tracer uptake consistent with a hyperfunctioning intrathyroidal cystic parathyroid adenoma. Ultrasound-guided sampling confirmed markedly elevated cyst fluid PTH. Definitive management with left hemithyroidectomy at 23 weeks' gestation resulted in normalisation of biochemistry and an uncomplicated term delivery. This first reported case of intrathyroidal cystic parathyroid adenoma in pregnancy localised with [¹⁸F]fluorocholine PET/CT highlights the diagnostic challenges of primary hyperparathyroidism in pregnancy and supports carefully justified, dose-optimised functional imaging when conventional modalities are inconclusive and surgical decision-making depends on accurate localisation.