Saturday, March 3, 2012
MCQ 4
A 45 year old female develops swelling in her neck and diarrhea. X-ray
shows dense calcification in her thyroid. A nuclear scan of her thyroid
detects a cold nodule that does not concentrate radio-iodine. Her doctor
does serum assays for several hormones. After the hormone is assayed,
he tells her general practitioner that the patient probably has has
medullary carcinoma of the thyroid since one of the hormones is markedly
?What hormone did the doctor most likely assay
a) thyroid stimulating hormone
b) thyroid hormone
c) calcitonin
d) Antidiuretic hormone
e) parathyroid hormone
b) thyroid hormone
c) calcitonin
d) Antidiuretic hormone
e) parathyroid hormone
:Correct Answer
c) calcitonin
:Explanation
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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