Binds ~90% of circulating IGF-1 — the IGF-1:IGFBP-3 molar ratio refines interpretation of GH-axis activity.
IGFBP-3 is the main circulating carrier of IGF-1 — about 90% of IGF-1 circulates bound to IGFBP-3, which extends its half-life and modulates tissue availability. The IGF-1:IGFBP-3 molar ratio gives more information than IGF-1 alone: a high ratio suggests more "free" (bioactive) IGF-1, which matters in some clinical contexts.
Like IGF-1, IGFBP-3 rises with GH stimulation and falls with age. It is also used in pediatric GH deficiency workup and in some oncology risk assessments.
IGFBP-3 is influenced by: GH status (moves in parallel with IGF-1), nutritional status (falls with protein restriction or illness), age (declines from peak in young adulthood), and directly by GH secretagogues.
Optional add-on when running tesamorelin, HGH, or MK-677 and wanting a more complete GH-axis picture. Not all users order it — IGF-1 alone is usually sufficient. When ordered, trend at baseline and 6 weeks alongside IGF-1.
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Start tracking →Informational only — not medical advice. Reference ranges vary by lab and individual context. Work with a licensed provider to interpret your specific results.