When Diarrhea Tells a Deeper Story: A Curious Case of Metastatic Medullary Thyroid Carcinoma

Chronic diarrhea is characterized by an increased frequency of loose stools persisting for more than four weeks. Diagnosing chronic diarrhea requires a comprehensive evaluation that includes a detailed medical history, physical examination, and investigations guided by diagnostic clues. Diarrhea is often the presenting symptom of various underlying conditions. Medullary thyroid carcinoma is a rare neuroendocrine tumor derived from the parafollicular C cells of the thyroid gland. It can occur sporadically or be inherited as a part of a syndrome, such as multiple endocrine neoplasia type 2. Chronic diarrhea can be a presenting symptom of medullary thyroid carcinoma and should always be considered in the differential diagnosis, particularly if the patient has a palpable thyroid nodule. This case report describes a 65-year-old woman with a rare metastatic medullary thyroid carcinoma who presented with chronic diarrhea and a palpable thyroid mass. Fine needle aspiration cytology and histopathological examination of the thyroid gland confirmed a diagnosis of medullary thyroid carcinoma. The diagnosis was also supported by increased calcitonin and carcinoembryonic antigen levels, which can be used for prognostication and monitoring response to therapy. She was treated with a tyrosine kinase inhibitor, which substantially alleviated her diarrhea, but she succumbed eventually to metastatic complications.