Effects of long-term oral administration of levothyroxine sodium on serum thyroid hormone concentrations, clinicopathologic variables, and echocardiographic measurements in healthy adult horses.

This research studied the effects of long-term ingestion of levothyroxine sodium on serum thyroid hormone levels, thyroid gland function, clinical results, and cardiac measurements in healthy adult horses. It found that while there were certain changes, there were no adverse effects on the horse’s health.

Methodology

• The study was conducted on 6 healthy adult mares.
• Each horse was administered with oral levothyroxine sodium (L-T(4)) at 48 mg/day for 48 weeks.
• Every four weeks, physical examinations were done and blood samples were taken for complete blood count, plasma biochemical analyses, and evaluation of serum total triiodothyronine (tT(3)) and thyroxine (tT(4)) concentrations.
• The activity of plasma creatine kinase MB and concentration cardiac troponin I, both important cardiac indicators, were also measured.
• Echocardiographic examinations were conducted before and at week 16, 32, and 48 of the treatment.

Results

• Overall, the horses’ average body weight decreased significantly over the treatment period.
• Fluctuations were observed in heart rate, but there was no evidence of a consistent pattern.
• While time effects were detected for certain clinicopathologic variables, the mean values stayed within the reference ranges.
• Cardiac troponin I was only detectable in 8 out of 24 plasma samples, with concentration ranging from 0.01 to 0.03 ng/mL, indicating no serious cardiac injury.
• Serum creatine kinase MB activity remained steady over time, indicating healthy muscle metabolism.
• Serum tT(4) concentrations saw an increased of 5.4-, 4.0-, and 3.7-fold at 16, 32, and 48 weeks in comparison to the pre-treatment value.
• Although some cardiac measurements changed significantly over time, these values remained within accepted reference ranges.
• Mean fractional shortening, a measurement of the heart’s pumping efficiency, was lower than the pretreatment mean value at week 16 and 32.

Conclusion

• The research concludes that, in horses, long-term oral intake of 48 mg of L-T(4)/d significantly elevated serum tT(4) concentrations.
• Despite these changes, there were no indications these fluctuations would adversely impact the horses’ health.