IGF-1 Interpreter (× ULN) — Acromegaly Screening and Control
IGF-1 Interpreter (× ULN) — Acromegaly Screening and Control
Enter an IGF-1 and your laboratory’s upper limit for the patient’s age and sex, and say whether this is a screening test or a patient on treatment. It gives the result as a multiple of the upper limit — the form the acromegaly guidelines use — reads it against the 1.3 × ULN diagnostic line and the treatment target, and flags the conditions that raise or lower IGF-1 without any change in growth hormone.
IGF-1 as a multiple of the upper limit
IGF-1 ÷ age-specific ULNIGF-1 520 ng/mL; illustrative upper limit 280 ng/mL; untreated, no confounders
IGF-1 as a multiple of the upper limit
- ULN
- your laboratory’s upper limit, for that assay, age and sex
- 1.3
- above this, with typical features, acromegaly is confirmed (2024 consensus)
- 1.0
- the treatment target is an age-normalised IGF-1, at or below the upper limit (Endocrine Society 2014)
Worked example
IGF-1 520 ng/mL; illustrative upper limit 280 ng/mL; untreated, no confounders
520 ÷ 280 = 1.86 × ULN
Above 1.3 × ULN: with typical features, this confirms acromegaly (2024 consensus)
Without typical features, confirm with an OGTT
The upper limit here is an illustration; use the one printed on the report
What IGF-1 × ULN means, by question
| Question | Line | Reading | Source |
|---|---|---|---|
| Screening | At or below 1.0 | Not raised | Laboratory interval |
| Screening | Above 1.0 to 1.3 | Equivocal: repeat on the same assay; OGTT | 2024 consensus; Endocrine Society 2014 (1.5) |
| Screening | Above 1.3 | Confirms acromegaly with typical features | 2024 consensus |
| On treatment | At or below 1.0 | Controlled (age-normalised IGF-1) | Endocrine Society 2014 (3.1) |
| After surgery | At or below 1.0 at 12 weeks | Surgical success | 2024 consensus |
Conditions that move IGF-1 without changing growth hormone status
| Direction | Condition | Effect on the reading |
|---|---|---|
| Lower | Malnutrition, anorexia nervosa | A normal IGF-1 is less reassuring |
| Lower | Liver disease or failure | IGF-1 is made mainly in the liver |
| Lower | Renal failure | As listed by the Endocrine Society 2014 |
| Lower | Oral oestrogen | First-pass hepatic effect; transdermal oestrogen much less |
| Lower | Poorly controlled diabetes | Portal insulin is needed for hepatic GH action |
| Lower | Hypothyroidism | Treat and repeat |
| Higher | Pregnancy | Placental growth hormone |
| Higher | Puberty and late adolescence | Use an interval for age and ideally pubertal stage |
Why IGF-1 is reported as a multiple of the upper limit
Growth hormone is secreted in pulses and spends much of the day near zero, so a single GH level says little. IGF-1, made mainly in the liver under GH stimulation and carried on binding proteins, integrates GH exposure over days, which is why it is the first test for acromegaly and the main marker of control on treatment. The 2014 Endocrine Society guideline recommends measuring it in anyone with typical features and in anyone with a pituitary mass.
The difficulty is the reference interval. IGF-1 peaks in adolescence and falls steadily through adult life, and different assays give different numbers for the same sample — the 2024 consensus says reference ranges are specific to each immunoassay, with the greatest differences at the highest values. So a raw IGF-1 cannot be compared with any fixed number, and this page carries no default interval. Dividing by the laboratory’s own upper limit for the patient’s age and sex gives a figure that can be compared: the multiple of the upper limit. The IGF-1 unit converter converts units, and shows how steeply the interval moves with age on one assay.
The 2024 consensus on acromegaly diagnosis states that in a patient with typical features an IGF-1 above 1.3 times the upper limit confirms the diagnosis. Between 1.0 and 1.3 the result is equivocal: repeat it on the same assay and consider an oral glucose tolerance test with GH sampling — see the OGTT growth hormone suppression interpreter. On treatment the target is an age-normalised IGF-1, and after surgery normalisation at 12 weeks defines success.
IGF-1 also moves for reasons unrelated to growth hormone. Malnutrition, liver and kidney failure, oral oestrogen, poorly controlled diabetes and hypothyroidism lower it; pregnancy and late adolescence raise it. A normal IGF-1 in someone who is malnourished or on oral oestrogen is therefore weaker evidence against acromegaly than it looks, and the page says so. For suspected deficiency rather than excess, IGF-1 is a poor test on its own; see the growth hormone stimulation test interpreter.
Frequently asked questions
What IGF-1 level indicates acromegaly?
IGF-1 is judged against the upper limit for the patient’s age and sex on that laboratory’s assay, not against a fixed number. The 2024 acromegaly consensus states that in a patient with typical features, an IGF-1 above 1.3 times the upper limit confirms the diagnosis. Between 1.0 and 1.3 times the result is equivocal and is repeated, with an oral glucose tolerance test if needed.
How do I calculate IGF-1 × ULN?
Divide the IGF-1 result by the upper limit of your laboratory’s reference interval for the patient’s age and sex, in the same unit. An IGF-1 of 520 ng/mL against an upper limit of 280 ng/mL is 1.86 × ULN.
What is the target IGF-1 on acromegaly treatment?
An age-normalised IGF-1 — at or below the upper limit for age (Endocrine Society 2014). The 2024 consensus adds that aiming for the mid to upper half of the interval can be considered in a clinically controlled patient to avoid growth hormone deficiency. On pegvisomant, IGF-1 is the only usable marker because GH levels rise.
What can cause a falsely low IGF-1?
Malnutrition, liver failure, renal failure, oral oestrogen, poorly controlled diabetes and hypothyroidism, as listed by the Endocrine Society 2014 guideline. In any of these a normal IGF-1 does not exclude acromegaly as reliably as usual.
Why doesn’t this page have a built-in IGF-1 reference range?
Because IGF-1 reference intervals are specific to each assay and change more than threefold across adult life. A built-in range would be wrong for most laboratories and most ages. The upper limit printed on the report, for the patient’s age and sex, is the only one that applies.
Related calculators
References
- Giustina A, Biermasz N, Casanueva FF, et al. Consensus on criteria for acromegaly diagnosis and remission. Pituitary. 2024;27(1):7–22. doi:10.1007/s11102-023-01360-1. — "In a patient with typical acromegaly features, insulin-like growth factor (IGF)-I > 1.3 times the upper limit of normal for age confirms the diagnosis"; equivocal results repeated on the same validated assay, with OGTT; IGF-1 normalisation at 12 weeks after surgery; reference ranges are assay-specific; calibration to 02/254.
- Katznelson L, Laws ER, Melmed S, et al. Acromegaly: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2014;99(11):3933–3951. — IGF-1 in suspected acromegaly and in any pituitary mass (1.1–1.3); OGTT for elevated or equivocal IGF-1 (1.5); falsely raised or lowered IGF-1 with pregnancy, late adolescence, oral oestrogen, hepatic and renal failure, hypothyroidism, malnutrition and poorly controlled diabetes; treatment target an age-normalised IGF-1 (3.1).
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/