Parturient Holstein cows were administered daily doses of supple-mental iodine as EDDI at 1.25, 2.50, and 5.0 mg. I/kg. of body weight and compared with contemporary controls in order to determine the effects iodine supplementation might have on vitamin A and thyroxine levels in the cow and her calf. Iodine was given from early stages of gestation until the cows were about 120 days into the following lactation. Blood samples were taken from the jugular vein of control and iodine-supplemented cows approximately 30 days prior to, on day of, and 30 days following calving. Plasma samples from calves were taken on day of calving. All plasma was analyzed for vitamin A, thyroxine, and total iodine content. Results of iodine administration on plasma vitamin A or thyroxine concentration of cattle gave no differences between diet groups, while plasma iodine increased with increased iodine administration. The high levels of iodine had no noticeable effects on plasma vitamin A levels in the calves. Higher thyroxine levels were found in calves from control cows, as they were almost twice the levels of the group supplemented with the highest level of iodine. Stage of gestation was found to affect vitamin A, thyroxine, and iodine levels in the cow. Plasma vitamin A and thyroxine values dropped to minimums on day of calving and then increased to almost normal (the value obtained 30 days prior to calving) by 30 days after calving. The plasma iodine level rose to a high on day of calving and then declined to a value close to the pre-calving level in the days following. Rats on a low vitamin A diet were supplemented with 0 and 250, 500, or 1000 ppm of iodine for 9 weeks and then killed. Heavier livers were found in the high iodine group than in controls. Differences in the concentration of liver vitamin A or plasma vitamin A due to the iodine feedings were not statistically significant. Differences in liver vitamin A per gram of tissue approached significance with the control group storing slightly less than the iodine groups. Rat plasma iodine, as in the cow, increased with the increasing levels of iodine fed. It was concluded from these studies that the feeding of excess iodine does not significantly affect vitamin A concentration in the plasmas or livers of rats or in the plasma of cows and their neonatal calves. Also iodine supplementation does not affect plasma thyroxine concentration in mature cattle. Iodine supplementation did significantly lower plasma thyroxine in calves, but it was not much lower than average thyroxine values reported in the literature for the normal calf. The study also confirmed the trends in plasma vitamin A, thyroxine, and iodine levels observed by other researchers during the peri parturient period of bovin
