TSH

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(a) TSH Upper Reference Limits

Over the last two decades, the upper reference limit for TSH has steadily

declined from ,10 to approximately

,4.0-4.5 mIU/L. This decrease reflects a number of factors including the

improved sensitivity and specificity of

current monoclonal antibody based immunometric assays, the recognition that

normal TSH values are logdistributed

and importantly, improvements in the sensitivity and specificity of the

thyroid antibody tests that are

used to pre-screen subjects. The recent follow-up study of the Whickham

cohort has found that individuals with

a serum TSH .2.0 mIU/L at their primary evaluation had an increased odds

ratio of developing hypothyroidism

over the next 20 years, especially if thyroid antibodies were elevated (35).

An increased odds-ratio for

hypothyroidism was even seen in antibody-negative subjects. It is likely

that such subjects had low levels of

thyroid antibodies that could not be detected by the insensitive microsomal

antibody agglutination tests used in

the initial study (207). Even the current sensitive TPOAb immunoassays may

not identify all individuals with

occult thyroid insufficiency. In the future, it is likely that the upper

limit of the serum TSH euthyroid reference

range will be reduced to 2.5 mIU/L because .95% of rigorously screened

normal euthyroid volunteers have

serum TSH values between 0.4 and 2.5 mIU/L.

Thyroid-Adrenal Connection: Information and Resources

http://www.bestweb.net/~om/thyroid

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