The effect of hyperthyroidism on circulating levels of free and
total igf-i and igfbps in rats.
Frystyk, J., H. Gronbak, C. Skjarbak, and A. Flyvbjerg.
Institute of Experimental Clinical Research, Medical Research,
Laboratories, Aarhus University Hospital, Aarhus, Denmark
APStracts 2:0122E, 1995.
Thyroid hormones are suggested to have a permissive role in growth
hormone (GH) and insulin-like growth factor-I (IGF-I) action, and a
specific effect on plasma levels of some of the GH-independent IGF
-binding proteins (IGFBPs). We have investigated the effect of
thyroxine (T4) administration on circulating levels of free and total
(extractable) IGF-I and IGFBPs in 8 weeks old rats treated with 0,
200, 400 and 600 Ng/kg T4, respectively. Serum free IGF-I was
determined by an ultrafiltration method, serum total IGF-I after
acid-ethanol extraction, and serum IGFBPs using Western ligand
blotting, which yielded 4 distinct molecular bands: two single bands
at 24 kDa and 30 kDa and a double band at 38 and 42 kDa (38-42 kDa).
After 13 days of hyperthyroidism serum total IGF-I and the high
molecular 38-42 kDa IGFBP were unchanged, whereas the 24 kDa IGFBP
and 30 kDa IGFBP had increased significantly (p < 0.05). Serum
free IGF-I was significantly (p < 0.05) decreased in animals
treated with 400 and 600 Ng/kg T4. In addition, free IGF-I correlated
inversely (p < 0.005) with the 24 kDa IGFBP, the 30 kDa IGFBP
and serum total T3. We conclude that hyperthyroidism in rats
increases the circulating low molecular IGFBPs and induces a
reduction in free IGF-I. This may provide an important regulation of
IGF-bioactivity by thyroid hormones.
Received 8 November 1994; accepted in final form 24 May 1995.
APS Manuscript Number E479-4.
Article publication pending Am. J. Physiol. (Endocrinol. Metab.).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 8 June 1995.