I have treated two horses in my life with thyroid disease. In 1972, a physician friend ask me to look at her horse. It had recently been purchased, and was noted on arrival to have an enlarged thyroid gland. I felt a 3-4 times enlarged nodular goiter.The blood testing of T4 was very low. I first thought the horse was hypothyroid, but after consultation with vet endocrine experts, I was told the horse's total T4 thyroid blood test will seem low by human standards, but the free T4 will be normal. The most likely cause was either a benign tumor, or enlargement secondary to either excess iodine in supplements, or iodine deficiency. There was no use of iodine supplements, so I diagnosed iodine deficiency.
Iodine salt lick was added to the horses diet, and the goiter markedly reduced in size.
The second horse had a large nodule, and after ultrasound studies , it was not consistent with a follicular tumor which is common in horses, but was more likely to be part of a diffuse goiter with a nodule. This time the owner did use kelp supplements,
which contained large amounts of iodine. Fortunately, with removal of the kelp, the nodular goiter reduced in size. Cancer of the thyroid is rare in horses.
Even horses can develop a goiter from excess or deficient iodine in their diet.
Humans in the USA, have a higher chance to develop excess iodine goiter from supplements, as iodine deficiency is rare in the USA.
My daughter's horse, Casey, does not have a goiter, thank God, or I would have had to treat my third horse in 31 years!
I will stick to human thyroid disease, as horses can not tell me what bothers them.
Dr.G.
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