Relationships between the thyroid and somatotropic axes in steers. I: Effects of propylthiouracil-induced hypothyroidism on growth hormone, thyroid stimulating hormone and insulin-like growth factor I.
The effects of propylthiouracil (PTU)-induced thyroid hormone imbalance on GH, TSH and IGF-I status in cattle were examined. In the first study, four crossbred steers (avg wt 350 kg) were fed a diet dressed with PTU (0, 1, 2 or 4 mg/kg/d BW) in a Latin square design with four 35-d periods. On day 29 in each period, steers were challenged with an intrajugular bolus of thyrotropin releasing hormone (TRH, 1.0 microgram/kg). Blood samples were obtained to assess the change in plasma GH and TSH as affected by PTU. Plasma IGF-I was measured from blood samples obtained before and after (every 6 hr for 24 hr) intramuscular injection of bovine GH (0.1 mg/kg, day 31). Doses of 1 and 2 mg/kg PTU increased plasma T4 (P < .01). At 4 mg/kg, PTU depressed T4 concentrations to 30% of control (P < .01). Plasma T3 linearly decreased with increasing doses of PTU (P < .01). Plasma TSH increased when PTU was fed at 4 mg/kg (P < .05) while the TSH response to TRH declined with increasing PTU (P < .02). Neither basal nor TRH-stimulated plasma concentration of GH was affected by PTU; the IGF-I response to GH tended to increase at the 1 and 2 mg/kg PTU (P < .01). In a second study 24 crossbred steers were fed PTU (1.5 mg/kg) for 119 d in a 2 x 2 factorial design with implantation of the steroid growth effector, Synovex-S (200 mg progesterone + 20 mg estradiol), as the other main effect. Basal plasma GH and IGF-I were not affected by PTU treatment. Synovex increased plasma concentration (P < .01) of IGF-I without an effect on plasma GH. The data suggest that mild changes in thyroid status associated with PTU affects regulation of T3, T4 and TSH more than GH or IGF-I in steers.
Elsasser
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U.S. Department of Agriculture, Ruminant Nutrition Lab, Beltsville, MD 20705.
Rumsey
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Norton
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D000818:Animals; D002417:Cattle; D002418:Cattle Diseases / Q000097:blood* / Q000139:chemically induced; D004338:Drug Combinations; D004958:Estradiol / Q000031:analogs & derivatives / Q000494:pharmacology; D013006:Growth Hormone / Q000097:blood*; D007037:Hypothyroidism / Q000097:blood / Q000139:chemically induced / Q000662:veterinary*; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008297:Male; D011374:Progesterone / Q000494:pharmacology; D011441:Propylthiouracil; D013963:Thyroid Hormones / Q000097:blood; D013972:Thyrotropin / Q000097:blood*; D013973:Thyrotropin-Releasing Hormone
D004338:Drug Combinations; D013963:Thyroid Hormones; D011374:Progesterone; D004958:Estradiol; D013973:Thyrotropin-Releasing Hormone; D007334:Insulin-Like Growth Factor I; D011441:Propylthiouracil; C025370:estradiol benzoate, progesterone drug combination; D013972:Thyrotropin; D013006:Growth Hormone