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Few patients have a high serum free thyroxine and a normal or high serum thyrotropin concentration

(November 2003)

The background of the study. Measurements of serum free thyroxine (T4) and thyrotropin (TSH) are reliable tests to identify thyroid dysfunction. An occasional patient has a high serum free T4 and a normal or high TSH concentration. The frequency of and explanation for this combination of results was determined in this study.

How the study was done. Serum TSH and free T4 were measured in 7918 serum samples in the clinical laboratory of a large hospital. Samples in which the serum free T4 concentration was high and the serum TSH concentration was normal or high were selected for this study. These serum samples were reanalyzed, if possible, and the patient's records were reviewed.

The results of the study. The serum free T4 and TSH concentrations were high and normal or high, respectively, in 18 of the 7918 samples (0.2 percent), from 17 patients. The results in three samples were from analytic errors. Three patients had thyroid hormone resistance, and one had just started T4 treatment for hypothyroidism. Three samples were from two newborn infants, who have high serum TSH and free T4 concentrations for several days after birth. The results in eight samples could not be confirmed or explained.

The conclusions of the study. The combination of a high serum free T4 and normal or high serum TSH concentration is rare, and often cannot be explained.

The original article. Mitchell DR, Parvin CA, Gronowski AM. Rules-based detection of discrepancies between TSH and free T4 results. Clin Chim Acta 2003;332:89-94