Primary hypothyroidism in dogs is associated with elevated GH release

The pulsatile secretion patterns of GH were investigated in seven beagle bi tches by collecting blood samples every 10 min for 6 h during euthyroidism and 1.5 years after induction of primary hypothyroidism. Hypothyroidism was induced by surgical removal of the thyroid gland and subsequent destructio n of any remnant thyroid tissue by oral administration of sodium [I-131]iod ide. Some of the physical changes observed in the dogs with primary hypothy roidism mimicked those of acromegaly. During both euthyroidism and hypothyr oidism GH was secreted in a pulsatile fashion. The mean (+/- S.E.M.) basal plasma GH concentration was significantly higher (P=0.003) in the hypothyro id state (4.1 +/- 1.6 mug/l) than in the euthyroid state (1.2 +/- 0.4 mug/l ). Likewise, the mean area under the curve (AUC) for GH above the zero-leve l during hypothyroidism (27.0 +/- 10.0 mug/l x 6 h) was significantly highe r (P=0.004) than that during euthyroidism (11.7 +/- 2.0 mug/l x 6 h). The m ean AUC for GH above the baseline was significantly lower (P=0.008) during hypothyroidism (2.4 +/- 0.8 mug/l x 6 h) than during euthyroidism (4.5 +/- 1.8 mug/l x 6 h), whereas there was no significant difference in GH pulse f requency. The mean plasma IGF-I level was significantly higher (P<0.01) in the hypothyroid state (169 +/- 45 <mu>g/l) than in the euthyroid (97 +/- 15 mug/l). The results of this study demonstrate that primary hypothyroidism in dogs is associated with elevated basal GH secretion and less GH secreted in pulses. This elevated GH secretion has endocrine significance as illust rated by elevated plasma IGF-I levels and some physical changes mimicking a cromegaly. It is discussed that the increased GH release in hypothyroid dog s may be the result of the absence of a response element for thyroid hormon e within the canine pituitary GH gene and alterations in supra-pituitary re gulation.