Preliminary evidence of immune function modulation by thyroid hormones in healthy men and women aged 55-70 years.

A reciprocal relationship between the endocrine and immune system has been demonstrated under pathophysiological conditions. However, few studies have assessed the relationship between thyroid hormones and immune function in apparently healthy individuals. Therefore, to clarify our understanding of normal physiological endocrine-immune interactions this study aimed to examine the interrelationships between thyroid hormones and immunity in healthy individuals. Total triiodothyronine (T(3)), total thyroxine (T(4)) and markers of immune status were assessed in 93 free-living and apparently healthy individuals aged 55-70 years. T(3) and T(4) concentrations were determined by commercially available kits. Immune status was assessed using flow cytometry and biochemical markers. Statistical analysis was performed by partial correlation, controlling for age. Thyroid hormone concentration was positively associated with markers of inflammation (P<or=0.05), natural killer-like T cells (P<or=0.001), expression of interleukin-6 (IL6) by activated monocytes (P<or=0.05); percentage expression of memory T-lymphocytes (P<or=0.01), memory T-helper lymphocytes (P<or=0.05) and memory T-cytotoxic lymphocytes (P<or=0.05), and higher IL2 receptor density on CD3+T-lymphocytes (P<or=0.05). Thyroid hormone concentration was inversely associated with early lymphocyte apoptosis (P<or=0.05) and the ratio of naïve- to memory T-cytotoxic lymphocytes (P<or=0.05). The current study provides preliminary evidence of a role for T(3) and T(4), within normal physiological ranges, in the maintenance of lymphocyte subpopulations, and in mediating the inflammatory response. In conclusion, these findings highlight the potential implications of altered thyroid function in older individuals and the importance of future research examining thyroid-immune interactions.