Hemodynamic effects of thyroidectomy in sedentary horses.

The study explores the impacts of thyroidectomy, a surgical removal of the thyroid gland, on the bodily functions of horses in resting condition. The findings suggest that the procedure notably reduces heart rate, respiration rate, rectal temperature, and blood circulation, while increasing blood volume and plasma volume. Administering thyroid hormone supplements partially reversed these changes.

Research Methodology

• A total of 6 healthy aged Quarter Horse mares were part of the study. The horses were monitored for 5 months prior to the thyroidectomy and 4 weeks following it. Further monitoring was done for an additional 4 weeks after the provision of a thyroid hormone supplement.
• The supplement contained 2.5 microg of thyroxine per kg of body weight and 0.6 microg of triiodothyronine per kg, administered orally every 12 hours.
• The responses to thyroid-stimulating hormone (TSH) were measured before thyroid surgery and 4 weeks afterward.
• Resting rectal temperature (T), heart rate (HR), respiratory rate (RR), and body weight (BW) were also monitored on a daily basis.
• Monthly measurements were taken for cardiac output (Q), blood volume (BV), plasma volume (PV), standard electrocardiographic measures, systolic and right ventricular blood pressure, and HR responses after intravenous administration of isoproterenol and phenylephrine.
• The collected data was examined with the use of repeated-measures ANOVA.

Research Findings

• The effectiveness of thyroidectomy was confirmed through the minimal response to TSH 4 weeks post-surgery.
• Significant decreases were observed in resting HR, RR, T, Q, and beta-adrenergic responsiveness to isoproterenol after the thyroidectomy.
• The administration of the thyroid hormone supplement led to an increase in resting T, Q, and beta-adrenergic responsiveness, with the values not significantly differing from euthyroid values.
• Substantial increases were observed in BV and PV after the thyroidectomy, and these increases did not return to euthyroid values despite the administration of the supplement.
• Response to phenylephrine showed minimal difference between treatments.

Research Conclusions

• Thyroidectomy in horses led to decreased resting heart rate, respiratory rate, rectal temperature, cardiac output, and isoproterenol responsiveness.
• It also led to an increase in blood volume and plasma volume, as well as PQ interval and corrected QT interval for heart rate.
• Some changes induced by the surgery seemed only partially reversible upon the administration of thyroid hormone supplements.