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A careful physical exam, especially the thyroid,
should always be carried out, since the presence
of a goiter would make the diagnosis of thyroid
dysfunction more likely. Since the serum TSH is
now the first thyroid function test obtained in
patients suspected of thyroid dysfunction or in
the screening of older patients, or those at risk for
thyroid disease, an abnormally low value should
be pursued. In this regard, a level below the lower
limits of detectability in any given assay raises the
suspicion of thyrotoxicosis. A patient with a TSH
value below the normal range but still detectable
is probably not thyrotoxic and the test should be
repeated in a few months. Further studies are
warranted in patients whose serum TSH is undetectable
and who are not receiving thyroid hormone
therapy. We usually obtain some measure
of thyroid hormone levels, primarily a serum T 4 ,
free T 4 (or index), and total T 3 (or free T 3 ), and
thyroid TPO and Tg antibodies. If either of the
circulating free thyroid hormones is elevated then
the patient almost certainly has some form of
overt thyrotoxicosis. If circulating thyroid hormone
levels are normal, suggesting subclinical
hyperthyroidism, a serum TSH should be repeated
in a few months, since the suppressed TSH may
be a transient finding. If either or both of the thyroid
antibodies are positive, autoimmune thyroid
disease is present and would warrant more careful
follow-up, since the first and only abnormality
in early Graves’ disease may be a suppressed
serum TSH. Whether a serum thyroid stimulating
immunoglobulin (TSI) should be obtained in such
a patient is debatable, but we usually do not in
this situation.
Other tests that can help to clarify the presence and etiology of subclinical hyperthyroidism in patients whose serum TSH remains suppressed with normal circulating thyroid hormones are as follows, and not in any specific order. First, a blunted or absent serum TSH response to TSH-releasing hormone (TRH) would suggest a diagnosis of subclinical hyperthyroidism 15 . Second, a thyroid 123 I uptake and scan revealing areas of hyperfunction raises the possibility of autonomous nodule(s). Finally, a serum thyroglobulin (Tg) might prove to be of interest. If the serum Tg is in the upper normal range or elevated, this would suggest that the thyroid may indeed be hyperfunctioning and secreting excess Tg. On the other hand, if the serum Tg is in the low-normal range, endogenous thyroid hyperfunction is less likely and if the serum Tg is suppressed, then thyrotoxicosis factitia should be ruled out. |
