Serum thyroxine concentrations and pregnancy rates 15 to 16 days after ovulation in broodmares.
OBJECTIVE
To determine whether serum thyroxine (T4) concentration was associated with pregnancy rates 15 to 16 days after ovulation in mares and to determine whether thyroid hormone supplementation would enhance fertility in mares.


METHODS
Cohort study.


METHODS
329 clinically normal broodmares.


METHODS
Mares were examined 15 to 16 days after ovulation to determine whether they were pregnant; blood samples for determination of serum T4 concentration were collected at the same time. Sixty mares were receiving thyroid hormone supplementation prior to the study because of low serum T4 concentration (< 16 microg/dl) prior to breeding.


RESULTS
Serum T4 concentration ranged from 4.5 to 53.9 mg/dl. Forty (12%) mares had low (< 16 microg/dl) concentrations, 283 (86%) had normal concentrations, and 6 (2%) had high (> 45 microg/dl) concentrations. Two hundred thirty-one mares were pregnant 15 to 16 days after ovulation. A significant association between serum T4 concentration (low, normal, or high) and pregnancy (yes or no) was not detected, and logistic regression analysis indicated that serum T4 concentration was not significantly related to pregnancy. Of the 269 mares not receiving thyroid hormone supplementation, 187 were pregnant, and of the 60 mares receiving thyroid supplementation, 44 were pregnant. There was no significant relationship between thyroid hormone supplementation and pregnancy status.


CONCLUSIONS
Results suggest that serum T4 concentration in mares is not significantly associated with pregnancy 15 to 16 days after ovulation. Results also suggest that supplementation of mares that only have low T4 concentrations is not indicated or likely to be beneficial.
Gutierrez
Cynthia V
CV
Rood and Riddle Equine Hospital, PO Box 12070, Lexington, KY 40580-2070, USA.
Riddle
W Thomas
WT
Bramlage
Larry R
LR
D000818:Animals; D015331:Cohort Studies; D005260:Female; D006736:Horses / Q000097:blood / Q000502:physiology*; D010060:Ovulation; D011247:Pregnancy; D018873:Pregnancy Rate; D011270:Pregnancy, Animal / Q000097:blood*; D013974:Thyroxine / Q000008:administration & dosage / Q000097:blood*
D013974:Thyroxine