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 thyro idectomy and for an additional 4 weeks after administration of thyroid horm one supplement (2.5 mu g of thyroxine/kg of body weight, PO, q 12 h, and 0. 6 mu g of triiodothyronine/kg, PO, q 12 h). Responses to thyroid-stimulatin g 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 me asures, systolic and right ventricular blood pressure, and HR responses wer e determined after IV administration of isoproterenol and phenylephrine. Va riables were analyzed by use of repeated-measures ANOVA. Results-Complete thyroidectomy was confirmed by minimal response to TSH 4 w eeks after surgery. Resting HR, RR, T, Q, and beta-adrenergic responsivenes s to isoproterenol decreased significantly after thyroidectomy. Resting T, Q, and beta-adrenergic responsiveness increased after administration of sup plement and was not significantly different from euthyroid values. Blood vo lume and PV increased significantly after thyroidectomy but did not return to euthyroid values despite administration of supplement. Response to pheny lephrine was minimally different between treatments. Conclusions and Clinical Relevance--Thyroidectomy in horses caused decrease d resting HR, RR, T, Q, and isoproterenol responsiveness and increased BV, PV, PQ interval, and QT interval corrected for HR. Some of these surgically induced changes appeared to be partially reversed by administration of thy roid hormone supplement.