37 y.o. man with hypercalcemia (not actual, but plausible)
Most people have four parathyroid glands, two superior and two inferior. The superior parathyroid glands arise from the fourth branchial pouch along with the thyroid gland are seldom ectopic. The inferior parathyroid glands arise from the third branchial pouch along with the thymus and more commonly ectopic, usually in the mediastinum.
Currently hyperparthyroidism are diagnosed as a result of a routine serum screen that shows hypercalcemia. Many of these asymptomatic patients have smaller parathyroid adenomas than symptomatic patients. The other common presentation of hyperrathyroidism is renal failure, with hypocalcemia resulting in hyperplasia of the parathyroid glands (secondary hyperparathyroidism) which occasionally progresses to autonomous growth of one or more of the parathyroid glands (tertiary hyperparathyroidism).
Localization of the hyperplastic parathyroid glands prior to surgery is often helpful to the surgeons. Smaller ectopic parathyroid tissue may be seen on the scintigram. One approach uses combined Tc-99m pertechnetate and Tl-201 subtraction imaging. The rationale is that thallium avidly accumulates in the both the parathyroid and thyroid tissue, whereas Tc-99m pertechnetate accumulates only in the thyroid tissue. If the normalized Tc-99m pertechnetate activity is subtracted from a thallium image, the contribution from the parathyroid glands will be evident. The technique has a number of pitfalls. First the relative uptake of Tc-99m pertechnetate and Tl-201 may not be equal and constant in patients with thyroid pathology (e.g., multinodular goiter, follicular adenoma). Secondly, patient motion may cause misregistration of data with result in inaccurate interpretation.
More recently, investigators have espoused the use Tc-99m sestabmibi as a replacement for Tl-201 and TCO4-. The protocol involves comparison of the early (20-minute) image with the late (2hour) images. Tc-99m sestabmibi will wash out of the thyroid faster than it will from the parathyroid tissue. Detection of parathyroid adenomas is more accurate than detection of parathyroid hyperplasia. SPECT (single photon emission computed tomography) may help in differentiating between parathyroid adenoma and hyperplasia.
EDITOR'S NOTE: A false positive sestamibi scan can result from retained tracer in a thyroid nodule. Therefore, it is important to examine the patient's neck to help exclude this as an etiology for a "positive" scan.
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