Objective—To determine the effects of levothyroxine
sodium (L-T4) on serum concentrations of thyroid
gland hormones and responses to injections of thyrotropin-releasing hormone (TRH) in euthyroid horses.
Animals—12 healthy adult mares.
Procedure—8 horses received an incrementally
increasing dosage of L-T4 (24, 48, 72, or 96 mg of
L-T4/d) for weeks 1 to 8. Each dose was provided for
2 weeks. Four additional horses remained untreated.
Serum concentrations of total triiodothyronine (tT3),
total thyroxine (tT4), free T3 (fT3), free T4 (fT4), and thyroid-
stimulating hormone (TSH) were measured in
samples obtained at weeks 0, 2, 4, 6, and 8; 1.2 mg
of TRH was then administered IV, and serum concentrations
of thyroid gland hormones were measured 2
and 4 hours after injection. Serum reverse T3 (rT3) concentration
was also measured in the samples collected
at weeks 0 and 8.
Results—Treated horses lost a significant amount of
weight (median, 19 kg). Significant treatment-by-time
effects were detected for serum tT3, tT4, fT3, fT4, and
TSH concentrations, and serum tT4 concentrations
were positively correlated ( r, 0.95) with time (and
therefore dosage) in treated horses. Mean ± SD
serum rT3 concentration significantly increased in
treated horses (3.06 ± 0.51 nmol/L for week 8 vs 0.74
± 0.22 nmol/L for week 0). Serum tT3, tT4, fT3, and TSH
concentrations in response to TRH injections differed
significantly between treated and untreated horses.
Conclusions and Clinical Relevance—Administration
of levothyroxine sodium increased serum tT4 concentrations
and blunted responses to TRH injection in healthy
euthyroid horses. (Am J Vet Res 2005;66:1025–1031)