What Is the Definition of Subclinical Hyperthyroidism?
Subclinical hyperthyroidism is defined as a serum TSH concentration below the statistically defined lower limit of the reference range when serum FT4 and T3 concentrations are within their reference ranges.12 Other causes of a low serum TSH must be excluded. Subclinical hyperthyroidism may result from endogenous overproduction of thyroid hormone or intended, or inadvertent, overadministration of thyroid hormone. Among other causes of a low serum TSH concentration with normal concentrations of FT4 are delayed recovery of the pituitary TSH-producing cells during or after therapy for hyperthyroidism,25 normal pregnancy,26 various nonthyroidal illnesses (euthyroid sick syndrome),27-28 or the administration of dopamine,29 glucocorticoids,30-31 and possibly dobutamine.32 Although subnormal serum TSH concentrations are common in a variety of severe nonthyroidal illnesses, undetectable serum TSH concentrations (<0.01 mIU/L) are rare unless patients are receiving concomitant glucocorticoids (us
