SAT-823 - Tertiary Hyperparathyroidism from an Intrathyroidal Parathyroid Adenoma

Background: Hyperparathyroidism is excess secretion of parathyroid hormone (PTH) and can be classified as primary, secondary, or tertiary. Primary hyperparathyroidism is due to inappropriate autonomous PTH secretion resulting in hypercalcemia. Secondary hyperparathyroidism is a compensatory response to conditions that cause a relative deficiency in calcium from its physiologic set point. Tertiary hyperparathyroidism (THPT) results from prolonged secondary hyperparathyroidism leading to autonomous PTH secretion. Primary and tertiary hyperparathyroidism result in hypercalcemia and are associated with a hypertrophic autonomous parathyroid gland that may be visualized on imaging. Parathyroid glands are typically posterior to the upper and lower poles of the thyroid gland. Rare anatomic variance during fetal development results in intrathyroidal parathyroid gland in 1.4-3.2% of individuals. These can be mistaken as thyroid nodules. We present a unique case of an intrathyroidal parathyroid adenoma in a patient with THPT that was misdiagnosed as thyroid nodule.

Clinical

Case: A 31-year-old female with history of end stage renal disease (ESRD) on dialysis and long-standing secondary hyperparathyroidism leading to possible tertiary hyperparathyroidism presented for evaluation. Evaluation included labs which showed PTH 4225 pg/mL, calcium 10.8 mg/dL, and creatinine 7.8 mg/dL. Alkaline phosphatase was also noted to be markedly elevated at 1334 U/L. Thyroid US revealed a 21 mm nodule within the left lower pole of the thyroid. Parathyroid scan revealed persistent asymmetric increased activity in the area of the left lower thyroid lobe consistent with a thyroid nodule vs left inferior parathyroid adenoma. Repeat thyroid US confirmed a 22 mm hypoechoic nodule within the left lobe of the thyroid. Fine needle aspiration (FNA) of the lesion revealed atypia of undetermined significance on cytology. PTH needle washout was markedly elevated >350,000 pg/ml. Findings were consistent with intrathyroidal parathyroid adenoma. The patient was diagnosed with THPT from likely autonomous intrathyroidal parathyroid gland in the setting of longstanding secondary hyperparathyroidism from ESRD. She was referred to an otolaryngologist for evaluation for surgical management of the parathyroid adenoma.

Conclusion: While multiglandular hyperplasia is seen in the majority of patients with THPT, up to 30% of patients with THPT can have a single adenoma. We present a patient with THPT with a large parathyroid adenoma that, interestingly, was intrathyroidal. An intrathyroidal parathyroid should be considered in patients with autonomous parathyroid function, an apparent thyroid nodule, and otherwise negative localizing studies. FNA of the nodule with PTH washout may be beneficial in confirming diagnosis of these cases as it was in our patient.

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