The Effects of Thyroid Hormone on Cytochrome P450 and P-Glycoprotein Activity in Thyroid Cancer Patients

Detailed Summary

This study will examine whether and how levothyroxine (Synthroid, a synthetic thyroid hormone) affects the way the body handles other drugs. If levothyroxine does affect the metabolism of other drugs, the dose of those medications may need to be increased to enhance their action or decreased to avoid adverse reactions. Patients 18 years of age and older with thyroid cancer who are participating in NIH protocol #77-DK-0096 and are receiving long-term suppression therapy with levothyroxine may be eligible for this study. This is not a study of thyroid cancer or of potential new drugs to treat it. Thyroid cancer patients are being studied because their treatment regimen provides an opportunity to study drug metabolism while patients are both on and off levothyroxine therapy. Participants come to the NIH Clinical Center on two occasions: once while they are regularly taking their levothyroxine, and once while they are off the medication in preparation for their radioactive iodide diagnostic scan for the procedures outlined below. The time interval between the two clinic visits depends on whether the first visit is while the patient is on or off medication. Participants are asked to fast overnight before each visit and to abstain from certain foods and beverages for 48 to 72 hours before the visit. At each visit, patients undergo the following procedures: * Medication history, limited physical examination, and blood draw for laboratory tests, including a test to look for genes important in eliminating medications from the body. * Insertion of a catheter (thin plastic tube) into an arm vein for collecting blood samples. * Shave skin biopsy (optional) to explore how proteins in the skin that metabolize and transport drugs are affected by thyroid hormone. For this procedure, the skin is cleaned, numbed with medicine, and a small sample of the top layer is removed with a razor blade. The wound heals in 2 to 3 days. * Medication dosing. Participants take all of the following substances by mouth at the same time: 1) 200 mg of caffeine, a compound commonly found in chocolate, soda/pop, coffee, tea and non-prescription products to prevent sleep; 2) 30 mg of dextromethorphan, a non-prescription cough suppressant; 3) 40 mg of omeprazole, a prescription drug for heartburn and stomach ulcers; 4) 8 mg of midazolam, a prescription drug used to cause relaxation and drowsiness; and 5) 120 mg of fexofenadine, a non-sedating prescription antihistamine used to treat allergies. * Blood and urine sampling. Fifteen blood samples of about 5 mL (1 teaspoon) each are collected through the catheter and urine is collected over the next 24 hours to determine what happens to the test drugs in the body. Participants may resume their normal diet 4 hours after taking the study medications.

Study Details

Timeline

Interventions