Factors influencing in vitro tests of thyroid function in the elderly.

Abstract

AbstractDeterminations of erythrocyte or resin I131‐triiodothyronine (T3) uptake and the serum protein‐bound iodine (PBI) concentration were made in 39 elderly patients on a geriatric ward and (for controls) on 20 relatively healthy residents of an old people's home.Difficulties in interpretation of these two tests of thyroid function may arise from a quantitative or qualitative alteration in thyroxine binding to protein. Some circumstances in which these test findings were altered in old people in the absence of clinical evidence of thyroid abnormality included accidental hypothermia, infections and the ingestion of certain drugs commonly used in geriatric practice, e.g., anabolic steroids. Results of the resin T3 uptake test were also more variable in elderly hospital patients than in normal old or young people. The changes in thyroxine binding by protein during administration of anabolic steroids were not due to competitive binding but to a reversible reduction in the amount of thyroxine‐binding protein.When determinations of thyroid function by in vitro tests are questionable, the free‐thyroxine index (the product of the T3 uptake and the PBI level) gives a true assessment of thyroid status.