SAT-303 - An Unusual Destination: Thyroid Metastasis as A Late Manifestation of Renal Cell Carcinoma

Introduction:
Renal cell carcinoma (RCC) typically does not metastasize to the thyroid gland; however, when it does it presents a complex challenge to endocrine care in an effort to maintain adequate thyroid hormone levels.  With metastatic thyroid cancer status post thyroidectomy, these patients have a lifelong need for thyroid supplementation.  If these patients are simultaneously undergoing active treatment for their RCC, this may lead to further challenge in maintaining adequate thyroid hormone levels. 
Case Presentation:
This patient is an 83-year-old female with a past medical history of RCC with metastasis to the thyroid gland, chronic kidney disease, atrial fibrillation, hyperlipidemia, and hypertension.  She initially presented with a chief complaint of significant fatigue. The patient had a primary diagnosis of RCC which was found to be metastatic to the thyroid gland. To treat this, the patient underwent a two staged thyroidectomy, leaving them with the need for thyroid replacement therapy. However, given the fact that the patient’s primary malignancy was RCC, she needed to continue treatment with lenvatinib, which is known to alter thyroid function by decreasing the conversion of T4 to T3 and increasing reverse T3 levels by interfering with deiodinases. Despite being on a significant dose of levothyroxine, the patient continued to have endorsed lethargy. Laboratory studies showed that her TSH was > 49.60 uIU/mL and her free T4 was 0.5 ng/dL.   At the time of this laboratory study, the patient had already been on 200 mcg of levothyroxine daily and the decision was made to initiate liothyronine 5 mcg twice daily in addition to this.  The patient will need to undergo routine thyroid function tests, especially while continuing treatment with lenvatinib, and should be completed at intervals of approximately 4 weeks. Despite dose escalation, normalization of her TSH was difficult even in the setting of appropriate intake with no interfering substances.

Conclusion:  
It is not necessarily common that RCC metastasizes to the thyroid gland.  However, when it does, it presents a unique challenge to endocrine care.  Certain medications used for the treatment of RCC are known to lead to difficulty in normalizing TSH by reducing the conversion from T4 to T3.  In such patients, the addition of liothyronine may be beneficial in an effort to bolster thyroid function.  These patients should be followed closely with recommended interval thyroid function testing completed approximately every 4 weeks, especially while receiving active treatment for their RCC.

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