I will do my best to answer what I think your questions are, please let me know if I have missed anything you wanted to know. hCG is human chorionic gonadotrophin and is given to mares to stimulate ovulation, in order to coordinate closely with insemination. This drug mimics the naturally occurring hormone LH (luteinizing hormone) and should cause ovulation within 48 hours of administration in mares already in heat. Estrumate is a synthesized version of the hormone prostaglandin F2alpha, which causes the corpus luteum on the ovary (which would normally help to support a pregnancy and forms after ovulation) to regress and the cycle to start again, bringing the mare back into heat. We use this drug for "short cycling" mares so that we don't have to wait for an entire cycle to pass before breeding. Estrumate can also be used to abort accidental conceptions up to a certain fetal age. As far as shots that a pregnant mare should get, I assume you are talking about vaccinations. Mares should generally receive their yearly boosters about a month before their foaling date so that the increase in immunity is passed in the colostrum to the foal (so most owners give Flu/Rhino, EWT, WNV, maybe Botulism toxoid, PHF or Strangles. You can give your rabies vaccine at this time or wait until breeding time). You should speak to your vet about which vaccinations are important in your geographic area, as this will vary. Pregnant mares should also receive Pneumabort-K shots (EHV-1) at 5, 7 and 9 months of gestation to prevent EHV-1 abortion. Thyro-L supplementation is used for various reasons, for horses who have tested with low thyroid hormone levels (hypothyroid), for those with metabolic disease that need help losing weight
, and in other performance horses that the owners feel need an additional spark. So the duration of treatment depends on the use. If treating due to a low blood value, you can re-check values with a follow-up blood test a few months into treatment and if the values are normal, try to decrease the dose gradually, checking every few months. When used for weight loss for metabolic syndromes, you may need to use it for 6 months to see marked results. When used for other reasons, the time the horse is kept on the drug varies per horse, owner and vet. Hope this answers your questions, let me know if I can clarify anything.