In reading the charges and fines against trainer Steve Asmussen in the Nov. 24 issue of TDN, I was upset to read that the administration of the thyroid hormone, thyroxine, T4, (a thyroid hormone molecule with 4 attached iodine atoms), was being referred to as a thyroid “supplement”. A supplement, in the truthful sense of the word, is/are the building blocks, which when administered to an animal/human allows the body to make its own natural chemicals and structures. Adding iodized salt, table salt to a feed would be a supplement as the iodine in the salt would be available once absorbed by the body to be used to make tetra iodinated thyroxine, Thyroid Hormone, also called T4. When T4 is released from the thyroid gland it circulates through the blood stream to all cells where it is taken into the cell cytoplasm, de-iodinated one time to Tri-iodothyronine, T3, the metabolically active form of thyroid hormone at the cellular level, where it has effects on cellular metabolism of sugars, fats and proteins as well as on cell oxygen utilization. On cardiac/heart cells, thyroid hormone, T4/T3, is a STIMULANT.
To administer Thyroxine exogenously, (given from the outside vs being produced naturally on the inside by the thyroid gland, endogenous production) is to administer a DRUG and that should only be done if the animal is confirmed by TSH (thyroid stimulating hormone) testing to be Thyroid deficient. In such a case, the TSH level is elevated by the following negative feedback loop mechanism. ( When thryroid hormone production by the thryroid gland is deficient there is not enough circulating T4 in the blood stream to tell the hypothalamus of the brain to not release, TSHRH (thyroid stimulating hormone releasing hormone) from the hypothalamus to the pituitary gland, so under the influence of TSHRH, the pituitary gland then releases, TSH, or thyroid stimulating hormone, to circulate to the Thyroid gland, rev up its production of T4 which will normalize the blood deficiency previously existing and the T4 which is normalized will negatively feed back to the hypothalamus, telling it to stop TSHRH release, all is balanced.
Here is the problem. In the athlete, which has normal thyroid gland function, the gland is producing T4, plus when it is given exogenous T4, (drug),plus-plus, the horse is then subjected to the oxigenating (aerobic), and anerobic stresses of a RACE, what happens? The cardiac cell, heart cell, subjected to all this excessive metabolic demands may not be able to fill its Oxygen debt obligation, and a “heart attack” occurs. The horse drops dead on the track! Have we seen this before? My guess, is that we have? If this has truly not been documented to have occurred in the past, with continued practice of the administration of thyroxine to Euthyroid horses (horses with normal thyroid gland function), it will be documented. There is simply too much stress to be placed on any animal’s body to not have catastrophic consequences.
I’d conclude by acknowledging what I as a MD, (Board Certified in Internal Medicine and Dermatology), and Thoroughbred breeder and owner have discovered from “hanging around the barn and engaging in all forms of training conversations”. There is a significant measure of naivete amongst horseman surrounding the concept of giving a dietary supplement to improve health and performance, verse administering an actual drug for medically necessary reasons. To those who may not be as familiar with pharmacology, the words of Attorney Clark Brewster on pg 3, clm2, lns 18,19, “In addition, Brewster said, it needs to be established whether the administration of thyroid supplement counts as a “drug.”, absolutely perpetuates confusion over this topic and obstructs the goal of “cleaning up” the Thoroughbred industry’s image.
I hope my letter helps improve understanding on the topic and leads to better care and protection of the horses.