Which thyroid cancer is known to cause diarrhea and why?

Medullary thyroid cancer is associated with the secretion of calcitonin, which can lead to a variety of symptoms, including diarrhea. Calcitonin is a hormone produced by the parafollicular cells (C cells) of the thyroid gland, and elevated levels of this hormone can influence gut motility, potentially resulting in diarrhea. In medullary thyroid cancer, the abnormal proliferation of these C cells leads to excess production of calcitonin, which in turn can have gastrointestinal effects.

In contrast, the other types of thyroid cancer mentioned do not cause diarrhea through the mechanisms noted. Follicular thyroid cancer primarily produces thyroxine (T4) and does not typically lead to elevated calcitonin levels. Papillary thyroid cancer generally does not secrete significant amounts of calcitonin either, and its association with increased thyroglobulin does not lead to gastrointestinal symptoms like diarrhea. Anaplastic thyroid cancer tends to be more aggressive and does not typically cause diarrhea; elevated TSH levels are more related to the tumor's progression rather than inducing gastrointestinal symptoms. Hence, medullary thyroid cancer’s secretion of calcitonin is the key factor linking it to the symptom of diarrhea.