This study will evaluate the benefits and side effects of a new medication called Cinacalcet for treating patients with primary hyperparathyroidism associated with multiple endocrine neoplasia type 1 (MEN1) or type 2A (MEN2A). Patients with primary hyperparathyroidism have elevated levels of blood calcium caused by too much parathyroid hormone released by one or more parathyroid tumors. The parathyroids are small glands located in the neck. Most cases of primary hyperparathyroidism are due to a single overactive parathyroid gland, but in MEN1 and MEN2A, several glands are overgrown and overactive. Cinacalcet decreases the secretion of parathyroid hormone. Patients 18 years of age and older with primary hyperparathyroidism and MEN1 or MEN2A and who are not candidates for parathyroid surgery may be eligible for this study. Participants are admitted to the Clinical Center for 1 week blood and urine tests and imaging studies, and initiation of Cinacalcet treatment. They take the drug by mouth and have daily blood tests until the dosage required to achieve normal blood calcium levels is determined. Patients return to the hospital 2 weeks later for 1 week to evaluate the response to the drug and make any necessary adjustments. Treatment may continue for as long as 1 year with 1-week admissions every 3 months to monitor the benefits and side effects of Cinacalcet. Evaluations may include the following: * Blood and urine analyses. * Measurement of gastric acid secretion. For this test, a soft plastic tube is inserted into the nose or mouth and then swallowed and then gently removed about an hour later. * Injections of secretin, calcium and arginine into a vein and collection of blood samples to measure the responding increase in levels of gastrin, calcitonin and insulin, respectively. These tests are used to diagnose and monitor hormone secretion from endocrine tumors and are used in this study to assess the response to Cinacalcet treatment. * Radioisotope test to evaluate tumors of the endocrine organs. A radioactive substance injected into a vein is taken up by the endocrine tissue and the concentrated radioactivity is measured. * Imaging tests, such as MRI and CT, to detect or follow growing tumors in the pituitary, neck, and abdomen. CT is a special type of x-ray machine that visualizes tissues, such as thyroid or parathyroid tumors. MRI uses a magnetic field and radio waves to obtain pictures of different tissues in the head, neck and abdomen. * DEXA scan to assess bone density. This test uses standard low-intensity x-rays.
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