Thyrotropin-releasing hormone-induced growth hormone secretion in dogs with primary hypothyroidism

• Published in: Domestic animal endocrinology Vol. 34; no. 2; pp. 176 - 181
• Main Authors: Diaz-Espiñeira, M.M., Galac, S., Mol, J.A., Rijnberk, A., Kooistra, H.S.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.02.2008; [Amsterdam; New York]: Elsevier Science Pub. Co
• Subjects: Animals; Dog Diseases - blood; Dog Diseases - drug therapy; Dog Diseases - physiopathology; Dogs; Female; hormonal regulation; hormone secretion; Human Growth Hormone - blood; Human Growth Hormone - secretion; hypothyroidism; Hypothyroidism - blood; Hypothyroidism - drug therapy; Hypothyroidism - physiopathology; intravenous injection; L-thyroxine; Male; Pituitary; somatotropin; thyroid gland; Thyroid glands; thyrotropin; Thyrotropin - blood; thyrotropin-releasing hormone; Thyrotropin-Releasing Hormone - pharmacology; Thyroxine; Transdifferentiation; TRH; TSH; TSH; TRH; Thyroid glands; Pituitary; Transdifferentiation; Thyroxine
• ISSN: 0739-7240; 1879-0054
• DOI: 10.1016/j.domaniend.2007.02.001

Primary hypothyroidism in dogs is associated with increased release of growth hormone (GH). In search for an explanation we investigated the effect of intravenous administration of thyrotropin-releasing hormone (TRH, 10 μg/kg body weight) on GH release in 10 dogs with primary hypothyroidism and 6 healthy control dogs. The hypothyroid dogs had a medical history and physical changes compatible with hypothyroidism and were included in the study on the basis of the following criteria: plasma thyroxine concentration <2 nmol/l and plasma thyrotropin (TSH) concentration >1 μg/l. In addition, 99mTcO 4 − uptake during thyroid scintigraphy was low or absent. TRH administration caused plasma TSH concentrations to rise significantly in the control dogs, but not in the hypothyroid dogs. In the dogs with primary hypothyroidism, the mean basal plasma GH concentration was relatively high (2.3 ± 0.5 μg/l) and increased significantly ( P = 0.001) 10 and 20 min after injection of TRH (to 11.9 ± 3.5 and 9.8 ± 2.7 μg/l, respectively). In the control dogs, the mean basal plasma GH concentration was 1.3 ± 0.1 μg/l and did not increase significantly after TRH administration. We conclude that, in contrast to healthy control dogs, primary hypothyroid dogs respond to TRH administration with a significant increase in the plasma GH concentration, possibly as a result of transdifferentiation of somatotropic pituitary cells to thyrosomatotropes.