Secretion pattern of thyroid-stimulating hormone in dogs during euthyroidism and hypothyroidism

In as many as one third of dogs with primary hypothyroidism a plasma thyrot ropin (TSH) concentration within the reference range for euthyroid dogs is found. To determine whether this is due to fluctuations in the release of T SH, the plasma profiles of TSH were analyzed in 7 beagle bitches by collect ing blood samples every 10 min for 6 hr, both before and after induction of primary hypothyroidism, After induction of primary hypothyroidism, a 37-fo ld increase in mean basal plasma TSH concentration and a 34-fold increase i n mean area under the curve for TSH were found. Analysis by the Pulsar prog ram demonstrated pulsatile secretion of TSH in the hypothyroid state, chara cterized by relatively low amplitude pulses (mean [+/-SEM]) amplitude 41 +/ - 3% of basal plasma TSH level) and a mean pulse frequency of 2.0 +/- 0.5 p ulses/6 hr. In the euthyroid state, significant TSH pulses were identified in only 2 dogs, The mean basal plasma TSH level correlated positively (r = 0.84) with the mean amplitude of the TSH pulses, and correlated negatively (r = -0.88) with the TSH pulse frequency. The results of this study demonst rate pulsatile secretion of TSH in dogs during hypothyroidism and only smal l fluctuations in plasma TSH concentrations during euthyroidism, The findin gs also suggest that the low TSH values occasionally found in dogs with spo ntaneous primary hypothyroidism may in some cases in part be the result of ultradian fluctuations, (C) 2000 Elsevier Science Inc. All rights reserved.