INTRODUCTION OF T4 IMMUNOASSAY
L-Thyroxine (T4) is a hormone that is synthesized and stored in the thyroid gland. Proteolytic cleavage
of follicular thyroglobulin releases T4 into the bloodstream. Greater than 99% of T4 is reversibly bound
to three plasma proteins in blood - thyroxine binding globulin (TBG) binds 70%, thyroxine binding prealbumin
(TBPA) binds 20%, and albumin binds 10%. Approximately 0.03% of T4 is in the free,
unbound state in blood at any one time.
Diseases affecting thyroid function may present a wide array of confusing symptoms. Measurement of
total T4 by immunoassay is the most reliable and convenient screening test available to determine the
presence of thyroid disorders in patients. Increased levels of T4 have been found in hyper-thyroidism
due to Grave’s disease and Plummer’s disease and in acute and subacute thyroiditis. Low levels of T4
have been associated with congenital hypothyroidism, myxedema, chronic thryoiditis (Hashimoto’s
disease), and with some genetic abnormalities.