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Thyrotropin Levels in a Population with No Clinical, Autoantibody, or Ultrasonographic Evidence of Thyroid Disease: Implications for the Diagnosis of Subclinical Hypothyroidism

Programs in Epidemiology (T.E.H., S.D., L.O.) and Cancer Prevention (K.J.K.), Division of Public Health Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109; Division of Endocrinology and Metabolism (T.E.H.), University of Washington School of Medicine, Seattle, Washington 98195; and Departments of Epidemiology (S.D.) and Biostatistics (K.J.K.), University of Washington School of Public Health and Community Medicine, Seattle, Washington 98195

Address all correspondence and requests for reprints to: Thomas Hamilton, M.D., Ph.D., Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue N, M4-A830, P.O. Box 19024, Seattle, Washington 98109-1024. E-mail: tehamilton{at}aol.com.

Context: The current debate regarding whether to decrease the upper limit for the TSH reference range to 2.5 µIU/ml has considerable potential impact on the diagnosis and treatment of subclinical hypothyroidism worldwide.

Objective: We report an analysis of TSH distribution in a population with no evidence of thyroid disease, including a normal thyroid ultrasound.

Design: A subset of the Hanford Thyroid Disease Study cohort was used to examine the TSH distribution in a population having no evidence of thyroid disease, seronegative thyroid autoantibodies, no history of thyroid medications, and a normal thyroid ultrasound. The shape of the TSH distribution was compared with the Gaussian and lognormal distributions.

Setting: This study was performed in the general community.

Participants: Of 1861 Hanford Thyroid Disease Study participants with TSH measured by ELISA who also had thyroid peroxidase antibody measurements, 766 comprised the normal reference group 3 (NRG-3) with no evidence of thyroid disease, including no positive antibodies and normal thyroid ultrasound.

Main Outcome Measure: TSH was measured.

Results: The TSH distribution in the NRG (NRG-3) was right skewed and followed an approximate lognormal distribution. The best estimates of the 97.5th percentile, the percentage above 2.5 µIU/ml, and the percentage above 3.0 µIU/ml for TSH by 3rd generation immunochemiluminometric assay are 4.1 µIU/ml, 20% and 10.2%, respectively.

Conclusion: These results indicate that the TSH reference range should be narrowed and support a value of approximately 4.0 as the upper-reference limit.

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