Insulin-like growth factor I alters peripheral thyroid hormone metabolism in humans: comparison with growth hormone.
Level 2 - randomized trial
Small randomized comparative clinical trial in humans
PubMed 8664976 · doi:10.1530/eje.0.1340563
What was done
Eight healthy subjects received subcutaneous IGF-I (10 μg/kg/h for 5 days). Eight patients with combined growth hormone (GH) and thyroid-stimulating hormone (TSH) deficiency from acquired pituitary disease (maintained on oral L-thyroxine) received subcutaneous IGF-I (10 μg/kg/h for 7 days), GH (2 IU/m² four times daily), or both in combination. Circulating thyroid hormone levels (free T3, free T4, total T4, and TSH) were assessed before and after treatment.
What was found
The abstract reports no exact numerical values or confidence intervals. In healthy subjects, IGF-I increased free T3 and decreased TSH, while free T4 and total T4 were unchanged. In T4-supplemented GH- and TSH-deficient patients, IGF-I increased free T3 while T4 remained unchanged. Treatment with GH alone and GH combined with IGF-I induced a more pronounced increase in T3 than IGF-I alone.
Why it matters
This study demonstrates that IGF-I partially mediates the stimulatory effect of growth hormone on peripheral conversion of T4 to T3 in humans, though GH retains additional IGF-I-independent mechanisms.
Limits
The sample size is very small (total n = 16 across two cohorts). The abstract does not provide exact numerical values, variance, or p-values for hormone changes. Duration of exposure was short (5 to 7 days), and long-term metabolic outcomes were not evaluated.
Cited by
- supports Growth hormone promotes the conversion of T4 to T3.