LOW SERUM l‐T3 LEVELS IN THE ELDERLY SICK: PROTEIN BINDING, THYROID AND PITUITARY RESPONSIVENESS, AND REVERSE T3 CONCENTRATIONS

Abstract

SUMMARYThyroid function was studied in thirty‐five euthyroid geriatric inpatients to define the relative importance of T4 to T3 conversion or pituitary‐thyroid dysfunction as a cause of low serum T3 concentrations. A previously reported finding of low serum total T3 levels was confirmed, and was associated with a fall in mean unbound T3 to 5.23 pmol/l (6.70 in younger normal subjects) and a rise in unbound T4 to 52.7 pmol/l (36.4 in normals). However, the protein binding of T3 and T4 was weaker in the patients (e.g. per cent unbound T4 increased 1.4 times) despite a mean increase in serum immunoreactive TBG concentration of 30%. Mean serum reverse T3 concentration (rT3) was nearly doubled in the patients (0.42 nmol/l compared with 0.24 in normals) but the correlation between high rT3 and low T3 was not close, and in a fifth of patients low T3 was not associated with high rT3. The T3 response to TSH injection was greater in patients with low T3 levels, and the T4 response smaller. The overall effect of TSH injection was to restore the previously increased T4 to T3 ratio of the patients to normal. The TSH response to TRH was absent in about a third of patients, and delayed in about another quarter, especially, although not exclusively so, in patients with low serum T3. The data suggest that several factors are concerned in the pathogenesis of low serum T3 concentrations in the elderly in hospital. Among these may be a shift in T4 deiodination from T3 to rT3, and hypothalamo‐pituitary TSH dysfunction. But it appears that other undefined factors may be responsible (such as nutrition or steroid levels); the pathogenesis is probably not the same in all patients.