Subclinical Hyperthyroidism -- Just a Low Serum Thyrotropin Concentration, or Something More?

Since the introduction 10 years ago of assays for serum thyrotropin that are sensitive enough to distinguish between normal and low concentrations, the measurement of serum thyrotropin has become the single best test of thyroid function. The reason is that an increase in the serum thyroxine (T 4 ) or triiodothyronine (T 3 ) concentration sufficient to cause clinically apparent hyperthyroidism inhibits thyrotropin secretion substantially, just as a decrease in the serum thyroid hormone concentration sufficient to cause hypothyroidism increases it. Serum thyrotropin concentrations are usually undetectable in patients with overt hyperthyroidism. They may be detectable but low in patients . . .