Supplementation with T4 thyroid hormone is a very common treatment in metabolic syndrome, but does it work?
Before the description of equine metabolic syndrome and high serum insulin (hyperinsulinemia), it was assumed that overweight horses with laminitis had hypothyroidism. Blood tests did often reveal low thyroid hormone levels. However, surgically removing the thyroid gland to produce severe hypothyroidism did not cause weight gain or laminitis.
There was one study on thyroid supplementation in EMS that showed some improvement in insulin. However, they had to use doses high enough to actually make the horses hyperthyroid causing weight loss, and there were also diet changes made.
A PhD thesis by Dr. K.A. Chameroy, clearly answers any questions. “Levothyroxine induces weight loss and reduces neck circumference in horses with EMS when animals are kept on a weight maintenance diet, but does not influence glucose dynamics or insulin sensitivity.” See pages 74 to 84 of the reference for details.
In other words, it won’t lower insulin if you provide thyroid hormone supplementation.
Thyroid hormone levels are low in EMS because of euthyroid sick syndrome, ESS. In ESS, the body reduces thyroid hormone levels to lower metabolism and help preserve precious supplies of glucose for the brain and heart when there is a perceived shortage. In EMS, there isn’t actually a shortage of glucose but it can’t get into the insulin requiring muscle normally because of insulin resistance so the body thinks there is a shortage.
The low thyroid levels can contribute to lethargy, low energy, and depression. Supplementation to restore normal levels may be reasonable but it comes at a price. The supplemented hormone suppresses activity of the thyroid gland, making it difficult to come off supplementation when the EMS is controlled and thyroid hormone levels would normally rebound on their own.
Several common nutritional deficiencies can interfere with the thyroid. In addition to magnesium, trace minerals and iodine, selenium status is very important. Selenium is required to convert thyroid hormones to their active form.
For EMS horses with low thyroid function, be sure the thyroid is adequately supported nutritionally while you work on the diet and exercise to control EMS. Thyroid function will return when EMS is controlled.
Chameroy, K A,Diagnosis and Management of Horses with Equine MetabolicSyndrome (EMS)Syndrome (EMS), PhD diss., University of Tennessee, 2010 https://trace.tennessee.edu/utk_graddiss/871