Relationship between circulating thyroid hormones and cell-mediated immunity in immature male chickens

The objective of this experiment was to examine the relationship between circulating triiodothyronine (T3), thyroxine (T4) and cell-mediated immunity in immature male chickens. Three week old Single Comb White Leghorn male chicks were used as the experimental animals. In order to produce a wide range of circulating thyroid hormone concentrations, birds were divided into groups and received one of nine treatments including surgical thyroidectomy; 0.1% propylthiouracil (PTU) in the feed; 1 ppm T3 and 10 ppm T4 in the feed. The graft versus host (GvH) response and response to phytohemagglutinin (PHA) were measured at 6 weeks of age. Total and differential white blood cells were counted. Birds were bled, weighed, sacrificed, and the lymphoid organs were removed and weighed. Concentrations of T3 and T4 were measured in the birds from all treatment groups. There were positive correlations between thyroid hormones, mainly T3, and weights of thymus and spleen. T4 but not T3 was positively correlated with number of circulating lymphocytes. There were no significant correlations between circulating T3 and T4 and either PHA or GvH responses. It was concluded that physiological levels of thyroid hormones are needed to maintain normal weights of thymus and spleen and a normal level of circulating lymphocytes. Furthermore, we conclude that lower than physiological levels might be sufficient to maintain normal cell-mediated immunity. Finally, in order to enhance cell-mediated immune responses using thyroid hormones, different doses than were used might be required.