Abstract Partial thyroid destruction was produced in one member of each of nine pairs of identical-twin dairy heifers with single oral doses of 131I varying from 99 to 180 μCi per kilogram of body weight. Twin pairs were bred beginning two months after treatment. First-calving reproduction was normal in the thyroid-damaged heifers. During lactation the six heifers dosed with 144 to 180 μCi per kilogram averaged, as a percentage of their control mates, 7% thyroid iodine uptake, 28% thyroxine secretion rate, 31% plasma protein-bound iodine, 83% heart rate, and 99% body weight. Three heifers dosed with 99 to 111 μCi averaged for the same values, respectively, 86, 70, 72, 87, and 101% of their control twins. One pair of twins milked only two months; the others all completed 10-month lactations except two thyroid-damaged twins in which lactation ceased after six and seven months. Although thyroid-damaged twins initially appeared fatter than the controls, their milk fat tests averaged 82% of control tests during the first two months of lactation, after which there was no significant difference. Average measurements for control and thyroid-damaged heifers, respectively, were milk yield, 4,000 and 2,042kg; milk fat, 4.29 and 3.96%; solids-not-fat, 8.77 and 8.66%; and 4% fat-corrected milk, 4,186 and 2,032kg.