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*** LAB NEWS ***
7/20/99
THYROID TESTING
Thyroid testing continues to be one of the more confusing areas
of clinical chemistry. At University Hospital we perform a few thousand
thyroid-related tests each month. This is largely appropriate, since
thyroid disease is common, and can be suspected in a very wide range
of clinical situations. Frequently, however, too many tests, or
the wrong tests, are ordered. It is not clear whether this is always
due to specific test orders by physicians, or to varying interpretation
of vague orders such as "thyroid function tests."
Most experts now recommend a "TSH first" approach when
screening for thyroid disease. TSH is the first test to become abnormal
in primary hypo- or hyperthyroidism. It is also a relatively specific
test, because it compensates for binding protein changes, disproportionate
secretion of T4 and T3, etc. Of course, TSH secretion can be affected
by pituitary disease and some drugs.
A test of free T4 (fT4) can be regarded as a second-line test. It
is often unnecessary if a TSH screening test is normal, but should
be performed when the TSH is abnormal. We recommend simply ordering
"free T4," in which case our laboratory performs a direct,
automated immunoassay for free T4. Alternatively, you may order
"free thyroxine index" or "FTI," which is a
calculated index requiring two measurements—total T4 and T-uptake.
Both the fT4 and FTI tests usually indicate thyroid status, but
they are subject to occasional artifacts because the fraction of
free hormone is so low.
Other thyroid tests, such as total and free T3, should be regarded
as third-line tests—they are useful in special situations,
but not in initial testing for hypo- or hyperthyroidism. (see Klee,
"Clinical usage recommendations and analytic performance goals
for total and free triiodothyronine measurements," Clinical
Chemistry 1996; 42:155-59. Also see guidelines on the web from
the American
Association of Clinical Endocrinologists).
Remember that payors, notably the Federal Government, DEMAND that
reimbursed tests be specifically ordered by a healthcare provider.
Orders such as "TFT’s" or "thyroid function
tests" are too vague and should be avoided. Please consult
with us, or the Division of Endocrinology, if you have any questions.
Jay L. Bock, MD, PhD
Chief of Clinical Chemistry and Director, Clinical Pathology
x4-2600
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