Natural killer activity and thyroid hormone levels in young and elderly persons

Background: On the basis that (1) multiple interactions exist between the h ormonal and immune systems, and (2) aging is accompanied by changes in thyr oid hormone metabolism and responsiveness, we postulate that thyroid hormon es 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 vitr o the effects of triiodothyronine (T-3) on NK activity. Materials and Metho ds:Sixteen of the 47 healthy old people (mean age 64 +/- 5.2) were classifi ed as optimally healthy, and the remainder as 'almost healthy' (according t o the criteria of the Senieur protocol) [Ligthart et al., Mech Ageing Dev 1 984;28:47-55]; the mean age of the healthy young people was 23.3 +/- 2.3 ye ars. NK cytotoxic activity of peripheral blood mononuclear cells was assess ed using Cr-51 release from K562 target cells. The cutoff level for definin g low and high NK responses was set at a value of 20%. Serum concentrations of TSH, total thyroxine (T-4) and total triiodothyronine (T-3) were measur ed by radioimmunoassay. Results: NK activity in the 'optimally healthy' eld erly was high (mean 41 +/- 12%, SE), whereas 'almost healthy' subjects show ed low NK activity (mean 6 +/- 5%). Serum T-4 and TSH levels, but not T-3 c oncentrations were similar in both the young and old. We observed a signifi cant correlation (r = 0.53, n = 21, p < 0.05) between the serum total T-3 l evel and the NK activity in the elderly individuals. Under in vitro conditi ons exogenous T-3 significantly increased NK activity in the elderly subjec ts who had serum T-3 values at the lower end of the reference range. Howeve r, no effect of T-3 on NK activity was observed in peripheral blood mononuc lear cells obtained from either old or young individuals who had serum T-3 levels at the midpoint of the range. Conclusion: Decreased serum concentrat ions of total T-3 may contribute to low NK activity in the 'almost healthy' subgroup of the elderly. Copyright (C) 2001 S. Karger AG, Basel.