Fine-needle aspiration cytology (FNAC) of thyroid nodules commonly reveals the presence of oncocytic cells (or Hurthle cells) in a follicular neoplasm. Histological examination is necessary to determine the benign or malignant nature of the tumour. However, oncocytic cells are also normally present in the parathyroid glands.

A thyroid nodule was discovered on thyroid ultrasound in a woman with a history of left partial thyroid lobectomy. Fine-needle aspiration cytology revealed a follicular neoplasm comprising oncocytic cells (Hürthle cells). This woman also presented features of hyperparathyroidism with hypercalcaemia. ¹²³I/^99mTc-sestamibi and ¹⁸F-fluorocholine PET-CT scan revealed increased uptake over the remaining left thyroid lobe. Left lobectomy was completed together with thyroid exploration. Histological examination revealed a parathyroid adenoma in the residual thyroid tissue. Parathyroid hormone levels subsequently returned to normal.

Cytomorphological similarities are often observed between parathyroid and Hürthle cell thyroid tumours. The parathyroid rather than thyroid nature of the tumour must be strongly suspected preoperatively in the presence of hyperparathyroidism.