Hyperplastic goiter in two adult dairy cows.

Iodine excess and resultant hyperplastic goiter are well documented in neonatal ruminants, but little is reported on iodine excess in adult ruminants and associated histological changes of the thyroid gland. Two adult Holstein cows from a Michigan dairy herd that had lost several other animals had nonspecific clinical signs of illness and were submitted for necropsy. Thyroid glands of one of these 2 animals were grossly and markedly enlarged, and histologically, thyroid glands from both animals had regions of cystic nodular hyperplasia and follicular atrophy. Thyroid glands from both animals had markedly elevated iodine concentrations. Investigation into the potential source of excessive iodine on the farm revealed multiple sources of supplemental dietary iodine and probable uneven feed and mineral mixing. Based on the findings of this investigation, adult cattle could be susceptible to excessive doses of iodine. Possibility of previous iodine deficiency before supplementation period, with subsequent development and persistence of thyroid hyperplasia and cystic change, cannot be completely excluded. Current findings suggested that iodine excess in adult cattle can result in nodular hyperplastic goiter. Use of iodized salt in mineral supplements in adult dairy herds is common practice, and accidental excessive iodine supplement may be more common than reported. Recognizing gross and histological thyroid gland changes, consisting of concurrent cystic follicular hyperplasia, atrophy, and fibrosis should raise suspicion of iodine excess and/or prior deficiency in a cattle herd, and ancillary tests such as serum iodine measurements should be part of the diagnostic workup in suspected cases.