My dr told me to take 1mg dexamethason tab the night before my thyroid panel blood test. why do i have to take?

Inzanium is right. The dexamethasone is not related to the thyroid tests. The doc is testing for Cushing's syndrome (though not specifically Cushing's disease, which is one cause of Cushing's syndrome). In a healthy person, if you take dexamethasone at night then have blood taken in the morning, your cortisol (a hormone produced by the adrenal glands) should be undetectable. If cortisol is detected, that can indicate Cushing's syndrome. Cushing's syndrone causes weight gain, stretch marks, muscle weakness, high blood pressure and high blood sugar.

i would see another doctor for another opinion