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Thyroid function tests are a series of measurements of different aspects of
thyroid hormone presence in the blood stream.
A common panel of thyroid function tests consists of:
T4 (Thyroxine)
This is a measure of all of the active form of thyroid hormone in the
blood. 95% of the total is bound to plasma proteins, particularly thyroid
binding globulin (TBG).
It is elevated in hyperthyroidism and depressed in hypothyroidism. It is
usually slightly elevated during pregnancy because of increased levels of
TBG.
T3 (Triiodothyronine, T3-RIA) by radioimmunoassay
T3 is metabolically a more active form of thyroid hormone than T4, but
its' effects are of shorter duration.
It is elevated in hyperthyroidism and depressed in hypothyroidism.
T3 Uptake
This is a measure of the unbound throxine binding globulins in the blood
(unsaturated with thyroid hormone). The less thyroid hormone that is
present, the more unsaturated TBG is present.
This is an indirect measure of thyroid hormone levels. It has nothing to
do with T3, despite the name.
Free T3 (FT3, Free Triiodothyronine T3)
Free T3 is one of the determinations used to measure thyroid function. T3
is mostly bound to plasma proteins, and the remainder (FT3) is unbound.
It is elevated in hyperthyroidism and depressed in hypothyroidism.
Free T4 (FT4, Free Thyroxin)
Most thyroid hormone is bound to plasma proteins. Some (about 5%) is not
bound ("free") and available to serve a metabolic role. Free T4 is
the most accurate assessment of thyroid hormone levels, because it avoids
changes in plasma protein binding capabilities.
It is elevated in hyperthyroidism and depressed in hypothyroidism.
FTI (Free Thyroxine Index, FTI, T7)
The FTI is obtained by multiplying the (Total T4) times (T3 Uptake) to
obtain an index.
The FTI is considered a more reliable indicator of thyroid status in the
presence of abnormalities in plasma protein binding.
It is elevated in hyperthyroidism and depressed in hypothyroidism.
TSH (Thyroid Stimulating Hormone)
Because of the normal feed-back loop, TSH rises whenever the thyroid
gland fails to produce sufficient thyroid hormone.
For the same reason, whenever there is too much thyroid hormone present
in the serum, the brain responds by lowering the TSH.
It is elevated in hyperthyroidism and depressed in hypothyroidism.
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