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Back to Endocrinology Lab tests
Thyroid function tests
Three tests are commonly referred to as thyroid function tests:
Free T4, serum
Free T4 is the metabolically active fraction of the total T4 level
(the other fraction is bound to plasma protein) and is the precursor
of T3.
The T4 reflects the amount of thyroxine in the blood. If the
patient does not take any type of thyroid medication, this test is
usually a good measure of thyroid function.
Methodology
A blood sample is taken and serum is separated within 48 hours. T4
is done by RIA (radioimmunoassay) is the most used thyroid test of
all. Other methods used are fluorescent immunoassay and
chemiluminescent immunoassay. A resin T3 uptake (RT3u) should be done
to correct for certain medications such as birth control pills, other
hormones, seizure medication, cardiac drugs, or even aspirin that may
alter the routine T4 test.
Reference levels
- Newborns: 2.6 - 6.3 ng/dL (SI: 33.5 - 81.3 pmol/L)
- Adults: 0.8 - 2.7 ng/dL (10.3-35 pmol/L)
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T3, serum
Thyroxine (T4) represents 80% of the
thyroid hormone produced by the normal gland and generally represents
the overall function of the gland. The other 20% is triiodothyronine
measured as T3. Sometimes the diseased thyroid gland will start
producing very high levels of T3 but still produce normal levels of
T4. Therefore measurement of both hormones provides an even more
accurate evaluation of thyroid function.
T4 is the precursor of T3 and is less
bound to plasma protein (thyroid binding globulin) and hence is more
biologically active.
Methodology
A blood sample is taken and serum is
separated within 48 hours. T3 is done by RIA (radioimmunoassay). Other
methods include fluorescence polarization immunoassay (FPIA) and
fluorometric immunoassay.
Reference levels
TSH, serum
Normally, low levels (less than 5 units)
of TSH are sufficient to keep the normal thyroid gland functioning
properly. When the thyroid gland becomes inefficient such as in early
hypothyroidism, the TSH becomes elevated even though the T4 and T3 may
still be within the "normal" range. This rise in TSH represents the
pituitary gland's response to a drop in circulating thyroid hormone;
it is usually the first indication of thyroid gland failure. Since TSH
is normally low when the thyroid gland is functioning properly, the
failure of TSH to rise when circulating thyroid hormones are low is an
indication of impaired pituitary function. Sensitive TSH tests will
show very low levels of TSH when the thyroid is overactive (as a
normal response of the pituitary to try to decrease thyroid
stimulation). Interpretations of the TSH level depends upon the level
of thyroid hormone; therefore, the TSH is usually used in combination
with other thyroid tests such as the T4 RIA and T3 RIA.
Methodology
A blood sample is taken and serum is
separated within 48 hours. A diurnal rhythm exists with peak TSH
levels occuring at about 11 PM. Pituitary production of TSH is usually
measured by a method referred to as IRMA (immunoradiometric assay).
Reference levels
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