Thyroid function tests in acutely ill patients. Comparison of analogue based free thyroid hormone assays with free thyroxine index.

A prospective study of 100 acutely ill patients was carried out to assess the value of the free thyroxine (FT4) assay as a replacement screening procedure for the free thyroxine index (FTI). We found that the FT4 assay was significantly influenced by the albumin concentration, so that the number of follow-up tests required increased markedly. This was especially true at the low end of the FT4 range where the need for thyrotropin assays increased by 162%. The free triiodothyronine (FT3) assay was also shown to be albumin dependent. It is not useful to replace one set of difficulties due to protein binding with another, and overall it was concluded that it is not cost-effective to screen hospital patients for thyroid dysfunction using free hormone assays based on labelled analogue techniques.