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

Abstract

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 (T 4, 5 mg/d, IM for 5 d) followed by triiodothyronine (T 3, 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 T 4 and again on day 5 of T 3. PTU alone increased plasma thyroid stimulating hormone (TSH), decreased plasma T 4 and T 3 but had no influence on IGF-I. T 3, but not T 4, lowered plasma TSH, IGF-I and 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 T 3 (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 T 3 treatments continuing. Plasma IGF-I was depressed 29% by T 3. The incremental area under the three-d response curve was less (P<.03) in T 3 cattle. A growth trial was conducted in which twenty-four Angus × Hereford steers were injected daily with T 3 (2 mg/kg, bi-daily × 56 d) or implanted with Synovex-S (S) in a 2 × 2 factorial arrangement. Synovex increased empty body protein gain (EBPG) and plasma IGF-I 15.5 and 27.9% (P<.01), respectively; T 3 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 T 3 decreases plasma IGF-I, in part, through a diminished GH-responsiveness and anabolic treatments such as S can reverse the effects of excess T 3.