The presence of very high levels of iodine in the diet of sheep in late pregnancy is subsequently associated with very low levels of immunoglobulins (Ig) in the blood of the lambs following consumption of colostrum (Boland et al., 2005). It remains to be established what minimum concentration of iodine in the diets of pregnant sheep affects IgG absorption, and whether the levels of iodine found in commercial concentrate feeds have an effect. The mechanism by which excessive iodine in the diet of pregnant sheep affects the absorption of Ig in lambs also remains to be established. The aims of this study were to determine the maximum dose of dietary iodine for late pregnant sheep that avoids the inhibition of IgG uptake from a standard quantity of colostrum given to neonatal lambs, and to examine the effects on thyroid hormone concentrations.