Question: Is it safe to give your thyroid supplement to a horse already on Thyro L?

Dr. Kellon: When your horse is being given supplemental thyroid hormone, that outside source of hormone will suppress the thyroid gland so that it reduces or stops its own production of thyroid hormone.  Thyroid supporting supplements can only be helpful if the thyroid gland is trying to produce hormone.  It's not unsafe per se unless the horse is on a high dose of supplemental Thyro-L since the combination could then be providing too high a dose of iodine.
The time to use thyroid supporting supplements is when you are attempting to support thyroid function without the use of replacement hormones or when weaning the horse off of the replacement hormones. Weaning off Thyro-L should be done very slowly, over 6 weeks or more if the horse has been on it for a long time, to allow the horse's own hormone regulation and production time to adjust and return to normal.  Begin the supplement when dose has been reduced to about  half of the original amount.