FACTORS INFLUENCING IN VITRO TESTS OF THYROID FUNCTION IN THE ELDERLY

Abstract

Determinations of erythrocyte or resin }l31-triiodothyronine (T 3 ) uptake and the serum protein-bound iodine (PRI) 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 iuterpretation of these two tests of thyroid functiou 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 aud the ingestion of certain drugs commonly used iu geriatric practice, e.g., anabolic steroids. Results of the resin T 3 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 T 3 uptake aud the PRI level) gives a true assessment of thyroid status.Many old people who are euthyroid have a hypothyroid appearance. Consequently it is all the more important to recognise mild or moderate degrees of true hypothyroidism. The most convenient tests of thyroid function in the elderly are those based on the thyroidal uptake of radioactive iodine, the serum level of radioactive protein-bound iodine (PBp3!) or in vitro blood tests.The basal metabolic rate (BMR) is often difficult to perform in aged subjects, and serum cholesterol concentration and the electrocardiogram (ECG) are also subject to changes by other diseases.Of the in vitro blood tests, the chemically determined serum proteinbound iodine (PBI) level is the most reliable (1), particularly for hypothyroidism, but it is time-consuming and often requires the facilities of a specially equipped laboratory. The p31 triiodothyronine (T 3) erythrocyte uptake test is simple and fairly reliable in detecting changes from the euthyroid state. The original erythrocyte uptake test (2) requires carefully standardised