Thyroid replacement in thyroparathyroidectomized dogs.
The thyroparathyroidectomized dog is an important experimental model of hypoparathyroidism and has been widely utilized for acute and chronic studies of the physiologic role of parathyroid hormone on systemic and renal acid-base, electrolyte and vitamin D physiology. Despite widespread use of this model, the appropriate thyroid hormone replacement dose necessary for achievement of postoperative euthyroidism has not been established for this species. Accordingly, serum thyroxine (T4) concentration was measured prior to and following chronic thyroparathyroidectomy in dogs given thyroid hormone replacement at different doses and routes of administration: sodium levothyroxine, 2.4 micrograms/kg daily in one or two divided subcutaneous doses (group I, n = 9), 20 micrograms/kg daily in two divided oral doses (group II, n = 8), and a wide range of intravenous doses (group III, n = 3). Group I dosage was based on the reported T4 production rate in dogs and is slightly greater than the reported production rate in man. Group II dosage was based on published clinically derived estimates of required replacement amounts. With subcutaneous replacement (group I) serum T4 concentration decreased from a preoperative value of 1.80 +/- 0.20 to 0.60 +/- 0.10 microgram/100 ml (p less than 0.001) following thyroparathyroidectomy. With oral replacement (group II), serum T4 concentration after thyroparathyroidectomy was not significantly changed from control (1.88 +/- 0.22 vs. 1.66 +/- 0.43 microgram/100 ml). Group III studies revealed that both total and free serum T4 concentration could be normalized after thyroparathyroidectomy with an intravenous dose of 10 micrograms/kg daily. As found with subcutaneous administration, intravenous replacement with lesser amounts of T4 than 10 micrograms/kg daily (i.e. 2.5 micrograms/kg) resulted in significant decreases in serum T4 concentration from control.(ABSTRACT TRUNCATED AT 250 WORDS)