Although medullary carcinoma of the thyroid is rare, this disease generates considerable interest due to its unusual cellular origin from parafollicular C cells, ability to secrete calcitonin, and association with other endocrine adenomatosis. Pentagastrin-calcium stimulation continues to be the most valuable test for early diagnosis, in detecting recurrence of tumor, and for screening of family members. Total thyroidectomy along with central neck dissection is the treatment of choice. However, management of metastatic disease in the lateral neck remains controversial. We report three cases of medullary carcinoma of the thyroid to illustrate its various forms and the use of diagnostic tests.