 We plan to screen healthy volunteers using a blood test to identify their genetic pattern relating to the enzyme we are interested in. From this group, forty-five healthy volunteers will be recruited for the hormone-test. This test called the TRH test uses a hormone produced by the brain and stimulates the pituitary and thyroid gland. The response to this test will allow us to compare the function of the thyroid system in relation to the genetic pattern of the volunteer. 