The term subclinical hypothyroidism (SCH) was coined by Bastenie et al1 in 1967, and the entity has remained a subject of a heated debate among clinicians ever since. There appears to be agreement on the basic definition of SCH as the finding of a thyrotropin (TSH) level above the upper limit of the reference range in the presence of normal reference range levels of serum free thyroxine (FT4) and free triiodothyronine (FT3).2 The controversy relates in part to just what is the normal reference range for TSH level, specifically the upper limit of the range beyond which a diagnosis of hypothyroidism would obtain. That upper limit has been falling in recent years from values of 6.5 or 7.0 mIU/L down to 4.1 to 4.5 mIU/L in most laboratories. We have argued that the upper limit of the reference range will be yet lower when reference populations are corrected for underlying or occult thyroid disease, goiter, antithyroid antibodies, and a family history of thyroid disease. When this is done, the mean serum TSH ranges from 1.2 to 1.5 mIU/L and the upper limit for the 97.5th percentile is 2.5 mIU/L.3
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