Hemodynamic effects of thyroidectomy in sedentary horses

Objective - To investigate hemodynamic effects of thyroidectomy in horses at rest. Animals - 6 healthy aged Quarter Horse mares. Procedure - Horses were monitored for 5 months before and 4 weeks after thyroidectomy and for an additional 4 weeks after administration of thyroid hormone supplement (2.5 μg of thyroxine/kg of body weight, PO, q 12 h, and 0.6 μg of triiodothyronine/kg, PO, q 12 h). Responses to thyroid-stimulating hormone (TSH) were measured before and 4 weeks after thyroidectomy. Other variables monitored daily were resting rectal temperature (T), heart rate (HR), respiratory rate (RR), and body weight (BW). Monthly cardiac output (Q), blood volume (BV), plasma volume (PV), standard electrocardiographic measures, systolic and right ventricular blood pressure, and HR responses were determined after IV administration of isoproterenol and phenylephrine. Variables were analyzed by use of repeated-measures ANOVA. Results - Complete thyroidectomy was confirmed by minimal response to TSH 4 weeks after surgery. Resting HR, RR, T, Q, and β-adrenergic responsiveness to isoproterenol decreased significantly after thyroidectomy. Resting T, Q, and β-adrenergic responsiveness increased after administration of supplement and was not significantly different from euthyroid values. Blood volume and PV increased significantly after thyroidectomy but did not return to euthyroid values despite administration of supplement. Response to phenylephrine was minimally different between treatments. Conclusions and Clinical Relevance - Thyroidectomy in horses caused decreased resting HR, RR, T, Q, and isoproterenol responsiveness and increased BV, PV, PQ interval, and -T interval corrected for HR. Some of these surgically induced changes appeared to be partially reversed by administration of thyroid hormone supplement.