SAT-408 - An Unlikely Culprit: Renal Cell Carcinoma Metastasizing to the Thyroid with Thyrotoxicosis

Metastatic disease to the thyroid is uncommon, accounting for 1-3% of all thyroid malignancies, with renal cell carcinoma (RCC) representing 48% of these cases. The thyroid’s rich arterial blood flow, high iodine concentration, and increased oxygen saturation are theorized to limit neoplastic growth, making metastasis rare. When RCC metastasizes to the thyroid, patients are typically euthyroid. We report a unique case of RCC metastasizing to the thyroid, causing infiltrative thyroiditis with thyrotoxicosis as the initial presentation of malignancy.

A 62-year-old male with coronary artery disease, type II diabetes, hypertension, and hyperlipidemia presented with an incidental 1.5 cm right thyroid nodule and thyromegaly on CT imaging. Laboratory evaluation revealed suppressed TSH (0.07 mIU/L), elevated free T4 (2.1 ng/dL), and the patient expressed a 40lb weight loss but no other symptoms of hyperthyroidism. Thyroid stimulating immunoglobulin and thyrotropin receptor antibodies were negative. Thyroid ultrasound demonstrated marked thyromegaly (right lobe 9.5 x 3.8 x 3.6 cm, left lobe 6.9 x 4.0 x 3.4 cm) with a right lobe 1.5 cm isoechoic solid nodule (TR3) and a 1.5 cm left lobe hypoechoic solid nodule (TR4). The patient underwent a thyroid uptake scan showing low uptake, consistent with subacute thyroiditis. Repeat ultrasound showed the left lobe nodule to be a benign colloid cyst (TR1), but the right lobe nodule enlarged to 1.8 cm (TR4). Fine needle aspiration of the right nodule reported benign cytology. With the development of compressive symptoms and positive Pemberton sign, a total thyroidectomy was performed. The histological studies unexpectedly revealed metastatic clear cell renal carcinoma. Subsequent abdominal MRI identified a 6.6 cm exophytic right renal mass, representing the primary malignancy. Nivolumab and ipilimumab were initiated with plans to undergo cytoreductive nephrectomy.

This case represents a rare presentation of RCC metastasizing to the thyroid with infiltrative thyroiditis, causing thyrotoxicosis. This is potentially the first reported case with this biochemical presentation, as prior case studies consistently report an euthyroid presentation. The mechanism likely parallels subacute thyroiditis, with tumor infiltration and inflammation damaging follicular cells, causing release of preformed thyroid hormone. The diagnostic challenge is evident in the false-negative FNA result. Metastatic RCC to the thyroid can be difficult to distinguish from primary thyroid lesions on cytology, with studies showing only 46% sensitivity for preoperative FNA diagnosis. Prognosis for isolated thyroid metastasis from RCC has a median survival of 3.4-4.7 years post thyroidectomy. This case emphasizes that thyroid nodules, particularly in patients with unexplained thyrotoxicosis and negative autoimmune markers, may represent metastatic disease.

*Unless otherwise noted, all abstracts presented at ENDO must not be released to the press or the public until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.*