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C Hodkinson, Haematology, Belfast City Hospital, Belfast, BT9 7AB, United Kingdom
E Simpson, Northern Ireland Centre for Food and Health (NICHE), University of Ulster, Coleraine, United Kingdom
J Beattie, Lipid & Redox. Regulation Group, Rowett Research Institute, Aberdeen, United Kingdom
J O'Connor, Northern Ireland Centre for Food and Health (NICHE), University of Ulster, Coleraine, United Kingdom
D Campbell, Northern Ireland Centre for Food and Health (NICHE), University of Ulster, Coleraine, United Kingdom
J Strain, Northern Ireland Centre for Food and Health (NICHE), University of Ulster, Coleraine, United Kingdom
J Wallace, Northern Ireland Centre for Food and Health (NICHE), University of Ulster, Coleraiine, United Kingdom

Correspondence: Clare Hodkinson, Email: clare.hodkinson{at}belfasttrust.hscni.net

A reciprocal relationship between the endocrine and immune systems 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 (T3), total thyroxine (T4), and markers of immune status were assessed in 93 free-living, apparently healthy individuals aged 55-70 yr. T3 and T4 concentration was 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 (P0.05), natural killer-like T (NKT) cells (P0.001), expression of IL-6 by activated monocytes (P0.05); % expression of memory T-lymphocytes (P0.01), memory T-helper lymphocytes (P0.05) and memory T-cytotoxic lymphocytes (P0.05), and higher IL-2 receptor density on CD3+ T-lymphocytes (P0.05). Thyroid hormone concentration was inversely associated with early lymphocyte apoptosis (P0.05) and the ratio of na¡ve T-cytotoxic to memory T-cytotoxic lymphocytes (P0.05). The current study provides preliminary evidence of a role for T3 and T4, 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.