The effects of triiodothyronine (T₃) treatments on general body growth, long bone growth, primary lymphoid organ development, antibody production, and serum growth hormone (GH) and thyroid hormone levels were examined in two dwarf strains (sex-linked dwarf—SLD, and autosomal dwarf—ADW) and in a normal-growing control strain (K) of White Leghorn chickens. One-day-old male chicks from each of these strains were assigned to either an untreated control group or to one of the groups receiving a T₃ supplement ranging from 0.01 to 1.0 ppm. General body growth and long bone growth were significantly (P < 0.05) stimulated only within the SLD strain by the intermediate T₃ dosages. The 1.0-ppm T₃ dosage level resulted in depressed body weights within both the K and ADW strains but produced no significant changes within the SLD strain. Thymic growth was significantly stimulated due to treatments of 0.1 ppm T₃ in the SLD strain (P < 0.05) and 1.0 ppm T₃ in both the SLD and ADW strains (P < 0.001 and P < 0.05, respectively). Bursal growth was significantly depressed (P < 0.05) at all T₃ dosage levels within the SLD strain while 0.01 and 0.1-ppm T₃ treatments resulted in significant bursal growth stimulation in the K and ADW strains, respectively. Concomitant with the depressed bursal growth, antibody production was significantly depressed (P < 0.05) within the SLD strain at the 1.0-ppm T₃ dosage level. Antibody production was not significantly affected by any of the T₃ treatments within the control K or ADW strains. Serum T₃ levels were significantly increased in all strains by the T₃ supplementation but thyroxine (T₄) serum levels were affected only within the SLD strain. The 0.01-ppm T₃ treatment resulted in a significant increase (P < 0.05) in serum T₄ levels within this strain and treatment group.