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.
Animals—329 clinically normal broodmares.
Procedure—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 µg/dl) prior to breeding.
Results—Serum T4 concentration ranged from 4.5 to
53.9 mg/dl. Forty (12%) mares had low (< 16 µg/dl)
concentrations, 283 (86%) had normal concentrations,
and 6 (2%) had high (> 45 µg/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
Conclusions and Clinical Relevance—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. (J Am Vet
Med Assoc 2002;220:64–66)