The effects of thyroid hormone deprivation and restitution on growth rate and pituitary and plasma GH levels have been studied in control and streptozotocin (STZ) diabetic rats. GH was measured by a specific RIA using the NIAMOD rat-GH kit. Male Wistar rats which had been surgically thyroidectomized for 30 days and which had been stopped growing were injected i.p. with either saline or STZ (4 mg/100 g b.wt). Eight days after saline or STZ, groups of thyroidectomized control (Thx-C) and thyroidectomized diabetic (Thx-D) rats were injected twice daily at 9 a.m. and 5 p.m. for 7 days with either saline or L-thyroxin (T4) 0.25, 0.5, or 1.0 μg/100 g b.wt/inj.
|Data: mean±S.D.||Δb.wt g/7 days||Pituitary GH (μg/mg)||Plasma GH (ng/ml)|
|Thx-D+0.5 μg T4||1.1±0.8||2±0.9||9.3±4.5|
|Thx-D+1 μg T4||2.2±1.3||9.4±4.5||12.9±7.5|
|Thx-D+2 μg T4||6.4±2.5||23.4±10.2||20.8±13.9|
|Thx-C+0.5 μg T4||11.6±3.2||32.8±9.3||38.4±10.5|
|Thx-C+1 μg T4||17.9±4.3||50.8±19.2||45.8±17.5|
|Thx-C+2 μg T4||21.3±5.6||69.4±14.9||41.4±14.1|
Present data show that when thyroid hormones are replaced in intensely hypothyroid control and diabetic rats both the pituitary GH concentration and the circulating GH levels increase. However, our data show that for comparable doses, T4 is less efficient on restituting the pituitary and plasma GH levels in the Thx-D than in Thx-C. Thus, it appears that normal insulin levels are required for biological action of thyroid hormones, at least on pituitary GH synthesis.