The consequences of subclinical thyroid disease
(serum TSH 0.1-0.45 µIU/mL or 4.5-10.0 µIU/mL) are minimal and current
guidelines recommend against routine treatment of patients with TSH
levels in these ranges, but thyroid function tests should be repeated at
6- to 12-month intervals to monitor TSH levels;8 however,
treatment of subclinical hypothyroidism is indicated in patients with
TSH levels >10.0 µIU/mL or in patients with TSH levels <10.0
µIU/mL in conjunction with goiter or positive for antithyroid peroxidase
antibodies (or both).9 In patients who are receiving
replacement therapy, the dose should be adjusted so serum TSH values
range from 0.3-3.0 µIU/mL. An exception is thyroid hormone replacement
treatment after thyroidectomy for differentiated thyroid cancer, in
which case, a mildly to moderately suppressed TSH level is generally
desirable.10 It is reasonable to consider serum TSH
measurement for pregnant women or women planning to become pregnant with
a family history of thyroid disease, prior thyroid dysfunction,
symptoms or physical findings suggestive of hypo- or hyperthyroidism, an
abnormal thyroid gland on examination, type 1 diabetes mellitus, or a
personal history of autoimmune disorder.11 Suggested upper
limit for the TSH reference range for pregnant women and preconception
is: first trimester - <2.5 µIU/mL, and 3.0 µIU/mL in the second and
third trimesters.10
Unsuspected increase in the level
of serum TSH is not uncommon in elderly subjects. A study by Sawin et al
found that 22 of 344 (5.9%) healthy persons older than age 60 had a TSH
level >10 µIU/mL; 10 of the 22 had low T4 and FT4 index. Elderly hypothyroid individuals may have minimal recognizable clinical symptoms of thyroid deficiency.11
TSH is the single most sensitive test for primary hypothyroidism. If
there is clear evidence for hypothyroidism and the TSH is not elevated,
hypopituitarism should be considered (secondary hypothyroidism).
TSH
levels have been elevated or inappropriately detectable for high
thyroid hormone levels in some patients with thyrotropin-secreting
pituitary adenomas. Delay in diagnosis of these tumors may lead to
visual compromise. The effects of such neoplasms can be misdiagnosed as
those of primary hyperthyroidism.
Until the late 1980s, TSH assays
were not sufficiently sensitive to distinguish hyperthyroidism from
euthyroid (normal) subjects. The new generation of ultrasensitive TSH
immunoassays have provided a far more effective diagnostic separation of
thyrotoxicosis from euthyroidism.
This assay has a sensitivity of 0.004 µIU/mL and meets all criteria as a third-generation TSH assay.