The unusually high sensitivity and specificity of the serum free thyroxine (FT4) index—the product of the total thyroxine (T4) and triiodothyronine (T3) uptake—makes this readily obtainable laboratory measurement a superb discriminator of thyroid dysfunction, even where the clinical findings of hypothyroidism or thyrotoxicosis are minimal or lacking.
In thyrotoxicosis, elevation of the FT4 index is an almost universal finding, occurring even before symptoms and signs are evident. In the unusual case of T3 thyrotoxicosis (T3 toxicosis), the FT4 index may not reflect the thyrotoxic state, but determination of the T3 concentration and calculation of the free T3 FT3 index enable the diagnosis to be made readily.
The T3 concentration and FT3 index are not good tests for hypothyroidism, since the results are normal in 30% to 50% of proved cases; but in the absence of pituitary disease, elevation of the thyroid-stimulating hormone (TSH) will presage the disorder,