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 primary hyperthyroidism.
Free T4 may more accurately measure the physiologic amount of T4.
Some patients who have been exposed to animal antigens, either in the environment or as part of treatment or imaging procedure, may have circulating antianimal antibodies present. These antibodies may interfere with the assay reagents to produce unreliable results.
This test cannot be used in patients receiving treatment with lipid-lowering agents containing dextrothyroxine unless therapy is discontinued for 4 to 6 weeks to allow the physiological state to become re-established prior to testing.