T4Total Thyroid Test

Total T4 levels offer a good index of thyroid function when the thyroid-binding globulin is normal and nonthyroidal illness is not present. Measurement of total T4 gives a reliable reflection of clinical thyroid status in the absence of protein-binding abnormalities. However, changes in binding proteins can occur that affect the level of total T4 but leave the level of unbound hormone unchanged. Caution and limitations: Twelve hours before this blood test do not take multivitamins or dietary supplements containing biotin or vitamin B7, which are commonly found in hair, skin, and nail supplements and multivitamins. In pregnancy, incomplete release of thyroxine (T4) from its binding proteins might result in falsely low total T4 levels. Therefore, total T4 should not be used as the only marker for thyroid function evaluation. Thyroid-stimulating hormone (TSH) may be better than thyroxine (T4) as the initial test of thyroid status. TSH is elevated in primary hypothyroidism. TSH is low in