What this test measures
Insulin-like Growth Factor 1 (IGF-1, also called somatomedin C) is produced mainly by the liver in response to growth hormone (GH) stimulation. It is the principal mediator of GH's growth-promoting and metabolic actions on tissues — and because it has a long half-life and a stable serum level (no diurnal or pulsatile variation), IGF-1 integrates 24-hour GH secretion in a single sample.
This makes IGF-1 the practical first-line test for both GH excess (acromegaly) and GH deficiency, in place of the unreliable random GH. Reference ranges are heavily age-dependent — IGF-1 peaks in adolescence and declines steadily into old age.
Why it matters
A normal age-matched IGF-1 makes both acromegaly and significant adult GH deficiency unlikely. A raised IGF-1 in an adult with appropriate clinical features (enlarging hands and feet, coarsening face, new dental gaps, sweating, headaches) raises strong suspicion of acromegaly — confirmed by failure of GH to suppress to < 1 ng/mL after an oral glucose load, then pituitary MRI.
In children, IGF-1 is used in the short stature workup. A low age-matched IGF-1 along with low IGF-binding protein 3 (IGFBP-3) in a child with abnormal growth velocity prompts GH-stimulation testing. IGF-1 is also used to titrate GH replacement in confirmed adult GH deficiency.
How to prepare
No fasting required. Sample can be drawn at any time of day — IGF-1 is stable. Stop biotin for 48–72 hours. Tell your doctor about all medications, especially recombinant GH if you are on it; the dose timing is irrelevant because IGF-1 reflects integrated activity, not the immediate dose.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Insulin-like Growth Factor 1 (IGF-1) (ng/mL)[1][2] | Highly age-dependent — examples: 10 yr: 110 – 565 · 20 yr: 116 – 358 · 40 yr: 74 – 196 · 60 yr: 51 – 200 · 80 yr: 38 – 156 | Low age-matched IGF-1 in a child with poor growth velocity suggests GH deficiency — confirm with GH stimulation. Low IGF-1 in an adult after pituitary surgery / radiotherapy is consistent with hypopituitarism. Also low in malnutrition, severe illness, uncontrolled diabetes, hypothyroidism and liver disease. | Raised age-matched IGF-1 with consistent clinical features (acral enlargement, coarse facial features, sweating, headaches) is highly suggestive of acromegaly. Confirm with an OGTT-suppressed GH and pituitary MRI. Pregnancy and adolescent growth spurt can raise IGF-1 physiologically. |
IGF-1 in clinical scenarios
| Scenario | IGF-1 | Next step |
|---|---|---|
| Adult with acromegaly features | Raised | OGTT with GH + pituitary MRI |
| Adult with treated pituitary tumour | Variable | Monitor disease activity / titrate replacement |
| Child with poor growth velocity | Low for age | GH stimulation test |
| Suspected adult GH deficiency | Low / low-normal | Stimulation test (insulin tolerance or glucagon) |
| Acromegaly post-surgery / SSA therapy | Falling | Reassuring treatment response |
Frequently asked questions
Why is IGF-1 used instead of growth hormone?
Growth hormone is secreted in unpredictable pulses — a single value can be normal high or normal low for the same person. IGF-1 has a stable level that reflects the average GH output over the previous 24 hours, so a single sample is informative.
My IGF-1 is just above the upper limit — do I have acromegaly?
Not necessarily. Mild raises can occur with adolescent growth, pregnancy and recent recombinant GH use. If suspicion of acromegaly is clinical (acral enlargement, sweating, dental change), the next step is an OGTT with GH suppression and pituitary MRI.
I am older and IGF-1 is "low" — should I be on GH therapy?
No. IGF-1 naturally declines with age, and age-matched reference ranges reflect this. Endocrine Society explicitly advises against GH supplementation in healthy older adults — there is no benefit and meaningful risk (insulin resistance, fluid retention, possibly cancer).
Is IGF-1 useful in monitoring my child's growth treatment?
Yes. On daily recombinant GH for confirmed deficiency, IGF-1 is checked every 3–6 months and the dose adjusted to keep IGF-1 in the upper half of the age-appropriate range without exceeding it.
Does fasting change IGF-1?
Not in a clinically meaningful way for a single sample. Severe malnutrition and prolonged catabolic states lower IGF-1, which is why it can also be low in chronic illness.
Will diabetes affect my IGF-1?
Yes — poorly controlled diabetes lowers IGF-1 because hepatic GH responsiveness is reduced. Get control before interpreting IGF-1 for other reasons.
My doctor wants IGF-1 with IGFBP-3 — why?
Most IGF-1 in blood is bound to IGF-binding protein 3 (IGFBP-3). In children with short stature, the combination of low IGF-1 + low IGFBP-3 is a stronger pointer to GH deficiency than IGF-1 alone.
Related Hormones / Endocrine tests
Tests commonly ordered alongside INSULIN LIKE GROWTH FACTOR 1, or that help interpret an unexpected result.
Sources & references
- Endocrine Society — Acromegaly Guideline · accessed 2026-05-30T00:00:00.000Z
- Endocrine Society — Adult GH Deficiency · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — IGF-1 Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — IGF-1 · accessed 2026-05-30T00:00:00.000Z
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