Primary hypothyroidism in dogs is associated with elevated GH release

Abstract

The pulsatile secretion patterns of GH were investigated in seven beagle bitches 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 destruction of any remnant thyroid tissue by oral administration of sodium [131I]iodide. Some of the physical changes observed in the dogs with primary hypothyroidism mimicked those of acromegaly. During both euthyroidism and hypothyroidism GH was secreted in a pulsatile fashion. The mean (±S.E.M.) basal plasma GH concentration was significantly higher (P=0.003) in the hypothyroid state (4.1 ± 1.6 μg/l) than in the euthyroid state (1.2 ± 0.4 μg/l). Likewise, the mean area under the curve (AUC) for GH above the zero-level during hypothyroidism (27.0 ± 10.0 μg/l × 6 h) was significantly higher (P=0.004) than that during euthyroidism (11.7 ± 2.0 μg/1 × 6 h). The mean AUC for GH above the baseline was significantly lower (P=0.008) during hypothyroidism (2.4 ± 0.8 μg/1 × 6 h) than during euthyroidism (4.5 ± 1.8 μg/1 × 6 h), whereas there was no significant difference in GH pulse frequency. The mean plasma IGF-I level was significantly higher (P<0.01) in the hypothyroid state (169 ± 45 μg/l) than in the euthyroid (97 ± 15 μg/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 illustrated by elevated plasma IGF-I levels and some physical changes mimicking acromegaly. It is discussed that the increased GH release in hypothyroid dogs may be the result of the absence of a response element for thyroid hormone within the canine pituitary GH gene and alterations in supra-pituitary regulation.