Increased immunological efficiency in young mice by short-term treatment with L-thyroxine.

Treatment of young mice with L-thyroxine increases their capacity to form hemolytic plaques after sheep erythrocyte immunization. Such an increment is independent from the quantity of antigenic challenge, from sex and strains variations and from the temporal relationship between timing of hormonal treatment and antigenic challenge. The increment seems to depend on an increased number of both T and B reactive cells, rather than on an augmented proliferation rate of antigen triggered cells. The thyroxine induced increase of T reactive cells requires the presence of a functional thymus. These data suggest that the maximal immune response obtained under physiological conditions does not correspond to the maximal potentiality which can be expressed through appropriate manipulation of the internal microenvironment and further confirm the relevance that thyroxine may have as an immunostimulant agent.