RESUMO
Primary hyperparathyroidism (PHPT) is the most common cause of hypercalcemia; patients are usually asymptomatic and the cause in 80-85% of cases is a single parathyroid adenoma (PA). Parathyroid adenomas arise from clonal expansion of tumor cells and may be located either posteriorly to the thyroid lobes or in ectopic sites. The incidence of intrathyroidal PAs varies from 1% to 6% and although uncommon, they pose certain diagnostic difficulties which may complicate treatment. The identification of the adenoma requires a combination of clinical evidence, imaging information and cytological findings due to the challenging distinction between thyroid and parathyroid lesions. We present the case of a patient with a large, partially cystic intrathyroidal parathyroid adenoma which was initially identified as a malignant thyroid nodule. We discuss the caveats that present in these rare cases and the important clinical and histological features that aid in the final diagnosis. In the case of our patient the cytological similarities between thyroid malignant cells and parathyroid cells, in combination with the negative sestamibi scan, resulted in a more invasive surgery than that a single parathyroid adenoma would require. Clinicians need to be vigilant in terms of the similarities between parathyroid and thyroid cytology in order to provide optimal patient care in these rare cases.