Thyroid function test. Investigation of low T4 (RIA) result; the differential
diagnosis of primary hypothyroidism from normal, and the differential
diagnosis of primary hypothyroidism from pituitary/hypothalamic hypothyroidism.
TSH is high in primary hypothyroidism. Low TSH occurs in hyperthyroidism.
Evaluation of therapy in hypothyroid patients, receiving various thyroid
hormone preparations: low values are found in states of excessive
thyroid replacement. Normal result on a sensitive TSH assay is acceptable
evidence of adequate thyroid replacement.
- 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 TSH level >10
mIU/L; 10 of the 22 had low T4 and FT4 index. Elderly hypothyroid individuals
may have minimal recognizable clinical symptoms of thyroid deficiency.10
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.
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