The only proven biological function of iodine is as a component of thyroid hormones that regulate energy metabolism and reproductive function. The major iodine deficiency symptom is goiter -- a swollen or enlarged thyroid gland in the neck. This should not be confused with the thymus gland in the neck on young animals, which is especially pronounced in Nubian kids but shrinks after they are several months old. Also, iodine deficiency causes reduced growth and milk yield, pregnancy toxemia, and reproductive problems such as late-term abortion, hairless fetus, retained placenta and weak kids. Most of the southern United States has adequate iodine in the soil, and most minerals and trace mineralized salts contain iodine. A number of areas in the northern United States are deficient in iodine due to soil geology.
Reference: Hart, S. 2008. Meat Goat Nutrition. Pages 58-83 in Proc. 23rd Ann. Goat Field Day, Langston University, Langston, OK.