Natural Killer Activity and Thyroid Hormone Levels in Young and Elderly Persons

Abstract

Background: On the basis that (1) multiple interactions exist between the hormonal and immune systems, and (2) aging is accompanied by changes in thyroid hormone metabolism and responsiveness, we postulate that thyroid hormones may be involved in the observed decrease in natural killer (NK) activity in a population of apparently healthy elderly subjects. The purpose of the study is to compare NK cytotoxic activity and serum concentrations of TSH and thyroid hormones in healthy old and young people, and to assess in vitro the effects of triiodothyronine (T₃) on NK activity. Materials and Methods: Sixteen of the 47 healthy old people (mean age 64 ± 5.2) were classified as optimally healthy, and the remainder as ‘almost healthy’ (according to the criteria of the Senieur protocol) [Ligthart et al., Mech Ageing Dev 1984;28:47–55]; the mean age of the healthy young people was 23.3 ± 2.3 years. NK cytotoxic activity of peripheral blood mononuclear cells was assessed using ⁵¹Cr release from K562 target cells. The cutoff level for defining low and high NK responses was set at a value of 20%. Serum concentrations of TSH, total thyroxine (T₄) and total triiodothyronine (T₃) were measured by radioimmunoassay. Results: NK activity in the ‘optimally healthy’ elderly was high (mean 41 ± 12%, SE), whereas ‘almost healthy’ subjects showed low NK activity (mean 6 ± 5%). Serum T₄ and TSH levels, but not T₃ concentrations were similar in both the young and old. We observed a significant correlation (r = 0.53, n = 21, p < 0.05) between the serum total T₃ level and the NK activity in the elderly individuals. Under in vitro conditions exogenous T₃ significantly increased NK activity in the elderly subjects who had serum T₃ values at the lower end of the reference range. However, no effect of T₃ on NK activity was observed in peripheral blood mononuclear cells obtained from either old or young individuals who had serum T₃ levels at the midpoint of the range. Conclusion: Decreased serum concentrations of total T₃ may contribute to low NK activity in the ‘almost healthy’ subgroup of the elderly.