Glycoprotein-secreting pituitary tumors

Published on 02/03/2015 by admin

Filed under Endocrinology, Diabetes and Metabolism

Last modified 02/03/2015

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Glycoprotein-secreting pituitary tumors

1. What are glycoprotein hormones?

2. Name two types of glycoprotein-secreting pituitary tumors and their secretory products.

3. Do pituitary tumors secrete only a single hormone?

4. Under what circumstances should a TSH-secreting tumor be considered?

5. Describe the differential diagnosis for patients with a transient increase in serum T4 and detectable or elevated TSH.

6. Describe the differential diagnosis for patients with a permanent increase in serum total T4 and detectable or elevated level of serum TSH.

7. What tests aid in the differential diagnosis of the patient with elevated serum total T4 and detectable or elevated TSH?

8. How can one distinguish between the hyperthyroid patient with thyroid hormone resistance and one with a pituitary tumor?

TSH tumors may secrete α-SU in excess of the whole TSH molecule. Therefore, the molar ratio of serum α-SU to TSH is increased in many patients with TSH tumors but is normal in those with thyroid hormone resistance. A thyrotropin-releasing hormone (TRH; protirelin) test is also helpful. Fewer than 20% of patients with a TSH tumor have a twofold increase in serum TSH after TRH administration, whereas those with resistance show a brisk response. T3 (triiodothyronine) suppression does not lower TSH in TSH-producing pituitary tumors but does do so in thyroid hormone resistance disorders. T3 suppression reduces Doppler color-flow and peak systolic velocity on thyroid ultrasound in most patients with thyroid hormone resistance, but not usually in patients with TSH-producing tumors. If a tumor is suspected, magnetic resonance imaging (MRI) of the pituitary should be obtained. Most TSH tumors (approximately 90%) are macroadenomas (i.e., ≥10 mm). Most microadenomas (<10 mm) are also visualized on MRI, but rarely, sampling of inferior petrosal sinus blood may be helpful in localizing a tumor. Dynamic MRI or somatostatin receptor scintigraphy (OctreoScan) is also useful. Long-term (2-month) administration of a long-acting somatostatin analog decreases serum free T4/T3 and TSH in patients with TSH tumors. Rarely, patients with TSH-secreting pituitary adenomas may have coexisting Graves’ hyperthyroidism or thyroid carcinoma.

9. Describe how to calculate an alpha subunit/TSH molar ratio.