Newly hatched White Leghorn male chicks were used in this study. Triiodothyronine (T₃; 0.1 or 1 ppm) and Thyrotropin Releasing Hormone (TRH; 1 or 5 ppm) were added to the feed for an 8-week period starting at hatch. A fifth group received the unsupplemented diet and served as controls. Peripheral blood lymphocytes from each treatment group were cultured in vitro with or without different mitogens (PHA, Con-A, or LPS), and the culture supernatants were tested for the presence of lymphokines (LK). The natural cell-mediated cytotoxicity (NCMC) assay was carried out with or without supernatant using the standard chromium (Cr⁵¹) release assay. Control untreated chicks were used as donors for effector cells and the P815 mouse mastocytoma was used as a target. Supernatant from in vivo 1 ppm T₃- or 5 ppm TRH-treated lymphocytes significantly suppressed NCMC (or cells mediating NCMC, e.g., NK cells). However, supernatant from 1 ppm T₃-treated, PHA-stimulated lymphocytes significantly enhanced NK cells cytotoxicity, while supernatant from 5 ppm TRH-treated lymphocytes with PHA stimulation tended to suppress cytotoxicity. These results provide evidence supporting a regulatory role of the hypothalamo-pituitary thyroid axis on lymphokine (LK) production.

The results also suggest that these hormones act on different subpopulation of lymphocytes, and therefore, the mediators released by them.