As we have seen from our overview of normal thyroid physiology, the thyroid
gland produces T4 and T3. But this production is not possible without
stimulation from the pituitary gland (TSH) which in turn is also regulated by
the hypothalamus's TSH Releasing Hormone. Now, with radioimmunoassay techniques
it is possible to measure circulating hormones in the blood very accurately.
Knowledge of this thyroid physiology is important in knowing what thyroid test
or tests are needed to diagnose different diseases. No one single laboratory
test is 100% accurate in diagnosing all types of thyroid disease; however, a
combination of two or more tests can usually detect even the slightest
abnormality of thyroid function.
For example, a low T4 level could mean a diseased thyroid gland ~ OR ~ a
non-functioning pituitary gland which is not stimulating the thyroid to produce
T4. Since the pituitary gland would normally release TSH if the T4 is low, a
high TSH level would confirm that the thyroid gland (not the pituitary gland)
is responsible for the hypothyroidism
If the T4 level is low and TSH is not elevated, the pituitary gland is more
likely to be the cause for the hypothyroidism. Of course, this would
drastically effect the treatment since the pituitary gland also regulates the
body's other glands (adrenals, ovaries, and testicles) as well as controlling
growth in children and normal kidney function. Pituitary gland failure
means that the other glands may also be failing and other treatment than just
thyroid may be necessary. The most common cause for the pituitary gland failure
is a tumor of the pituitary and this might also require surgery to remove.
The tests for thyroid gland are following:-
Thyroid Binding Globulin:
Most of the thyroid hormones in the blood are attached to a protein called
thyroid binding globulin (TBG). If there is an excess or deficiency of this
protein it alters the T4 or T3 measurement but does not affect the action of
the hormone. If a patient appears to have normal thyroid function, but an
unexplained high or low T4, or T3, it may be due to an increase or decrease of
TBG. Direct measurement of TBG can be done and will explain the abnormal value.
Excess TBG or low levels of TBG are found in some families as an hereditary
trait. It causes no problem except falsely elevating or lowering the T4 level.
These people are frequently misdiagnosed as being hyperthyroid or hypothyroid,
but they have no thyroid problem and need no treatment.
Measurement of Serum Thyroid Hormones: T3 by RIA:
As stated on our thyroid hormone production page, 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 by RIA. 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.