Relationships between the thyroid and somatotropic axes in steers. II: Effects of thyroid status on plasma concentrations of insulin-like growth factor I (IGF-I) and the IGF-I response to growth hormone.
Three studies assessed the effect of thyroid status on regulation of plasma IGF-I in cattle. First, four Angus-Hereford steers (av wt 345 kg) were fed 4 mg/d propylthiouracil daily for 35 d. With continued feeding of PTU steers were sequentially injected with thyroxine (T4, 5 mg/d, IM for 5 d) followed by triiodothyronine (T3, 2 mg/d, IM for 5 d). An injection of bovine pituitary growth hormone (GH, 0.1 mg/kg, IM) was given to each steer on day 35 of PTU, day 5 of T4 and again on day 5 of T3. PTU alone increased plasma thyroid stimulating hormone (TSH), decreased plasma T4 and T3 but had no influence on IGF-I. T3, but not T4, lowered plasma TSH, IGF-I and the IGF-I response to GH (P < .05). Next, twelve bull calves (av wht 167 kg) were divided equally into two groups. A control group was injected daily for five d with buffered saline; the experimental group was concurrently treated with T3 (5 mg/d, sc) for five d. Beginning the sixth day, all calves were injected with GH (0.1 mg/kg, IM daily) for three d with the respective buffer or T3 treatments continuing. Plasma IGF-I was depressed 29% by T3. The incremental area under the three-d response curve was less (P < .03) in T3 cattle. A growth trial was conducted in which twenty-four Angus x Hereford steers were injected daily with T3 (2 mg/kg, bi-daily x 56 d) or implanted with Synovex-S (S) in a 2 x 2 factorial arrangement. Synovex increased empty body protein gain (EBPG) and plasma IGF-I 15.5 and 27.9% (P < .01), respectively; T3 decreased EBPG and plasma IGF-I 13.9 and 15.1% (P < .07), respectively, in steers which maintained suppression in plasma TSH. The data support the conclusion that elevated T3 decreases plasma IGF-I, in part, through a diminished GH-responsiveness and anabolic treatments such as S can reverse the effects of excess T3.
Elsasser
T H
TH
U.S. Department of Agriculture, Ruminant Nutrition Laboratory, Beltsville, MD 20705.
Rumsey
T S
TS
Kahl
S
S
D000818:Animals; D002417:Cattle / Q000502:physiology*; D002418:Cattle Diseases / Q000378:metabolism; D004338:Drug Combinations; D004347:Drug Interactions; D004958:Estradiol / Q000031:analogs & derivatives / Q000494:pharmacology; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology*; D013006:Growth Hormone / Q000494:pharmacology*; D007037:Hypothyroidism / Q000378:metabolism / Q000662:veterinary; D007334:Insulin-Like Growth Factor I / Q000096:biosynthesis*; D016018:Least-Squares Analysis; D008297:Male; D015999:Multivariate Analysis; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism; D011374:Progesterone / Q000494:pharmacology; D011441:Propylthiouracil / Q000494:pharmacology; D014176:Protein Biosynthesis; D011897:Random Allocation; D013961:Thyroid Gland / Q000187:drug effects / Q000502:physiology*; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood / Q000494:pharmacology; D013997:Time Factors; D014284:Triiodothyronine / Q000097:blood / Q000494:pharmacology
D004338:Drug Combinations; D014284:Triiodothyronine; D011374:Progesterone; D004958:Estradiol; D007334:Insulin-Like Growth Factor I; D011441:Propylthiouracil; C025370:estradiol benzoate, progesterone drug combination; D013972:Thyrotropin; D013006:Growth Hormone; D013974:Thyroxine