UCLA Department of Medicine Clinical Insights

This case describes a 36-year-old woman who presents with a palpable left-sided thyroid nodule and laboratory evidence of dual endocrine dysfunction: subclinical hyperthyroidism and primary hyperparathyroidism). While thyroid autoantibodies were negative, imaging revealed a 2.85 cm complex nodule in the left inferior thyroid lobe.

Diagnostic challenges arose when a sestamibi scan and parathyroid CT failed to identify a distinct parathyroid adenoma, showing only a hyperfunctioning focus corresponding to the known thyroid nodule. The suspicion of an intrathyroidal parathyroid adenoma (IPA) was confirmed via fine-needle aspiration (FNA) with PTH washout, which returned a significantly elevated level of 549 pg/mL.

The patient underwent a left thyroid lobectomy. Histopathology confirmed a 1.0 cm parathyroid adenoma embedded within the thyroid tissue. Postoperatively, intraoperative PTH levels dropped by over 95%, and at the three-month follow-up, both TSH and calcium levels had normalized. This case highlights the importance of considering IPA in the differential diagnosis of concomitant thyroid nodules and hypercalcemia, illustrating that PTH washout remains a critical tool when localized imaging is ambiguous. Five-year follow-up confirms sustained biochemical remission.