Answer :USMLE step 2 Mcq 234:A 45 year old female develops swelling in her neck and diarrhea




Correct Answer: C

Medullary thyroid cancer is a malignancy of the thyroid parafollicular cells. Thyroid parafollicular cells normally produce the hormone calcitonin. A malignancy of these cells, therefore, can also produce calcitonin. Assay of calcitonin is a very good diagnostic test for medullary carcinoma of the thyroid. Thyroid stimulating hormone (choice a) is an anterior pituitary hormone and is not produced by the thyroid gland at all. A tumor of the parafollicular cells would not produce thyroid stimulating hormone. Thyroid hormone is not produced by the parafollicular cells, but rather by the follicular cells. A tumor of the parafollicular cells would not produce thyroid hormone.Antidiuretic hormone is normally produced by the hypothalamus and released by the posterior pituitary. ADH can be seen other types of tumors as ectopic hormone production and produce a paraneoplastic syndrome. However, a tumor of the parafollicular cells would not produce antidiuretic hormone To understand which mechanism is responsible for uptake in medullary thyroid carcinoma one needs to explore the subject further. This type of carcinoma arises from the neoplastic transformation of the C-cells of the thyroid. These cells are derived from the neural crest embryologically rather than the foregut endoderm as is the remainder of the thyroid. The tumor affects the upper third of the lateral lobe of the thyroid since this is where most of the C-cells are concentrated. Medullary carcinoma accounts for 10%% of thyroid neoplasms. It occurs sporadically in 80-90%% and is familial in 10-20% of cases. The familial type is transmitted as autosomal dominant and is associated with the MEN II or MEN III syndrome. Thyroid function tests are usually normal. Numerous hormones are produced by this neoplasm and these include calcitonin, ketocalcin, L-Dopa decarboxylase, carcinoembryonic antigen, serotonin, prostaglandins, ACTH, histaminase and substance p. It should be noted that the first four hormones are also secreted by normal C cells. Elevated calcitonin is related to the secretory diarrhea which occurs in 30% of cases. This particular hormone is the only sensitive indicator for the presence of tumor because it is the only one showing response to provocative or suppressive stimuli. Elevated histaminase correlates well with metastasis


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