IGFBP-3 Unit Converter
IGFBP-3 Unit Converter
Convert IGFBP-3 between mg/L, µg/mL, ng/mL and nmol/L, against age-matched adult intervals — and note that the molar figure is an approximation, because the circulating protein is glycosylated and its mass is not fixed.
IGFBP-3 converter
Mass ⇄ molarIGFBP-3 4.2 mg/L in an adult aged 18
The four units, and why the molar one is approximate
ng/mL = mg/L × 1000
nmol/L = ng/mL × 0.0348432
derived from a nominal molecular weight of 28,700 Da
- mg/L = µg/mL
- the unit almost every report uses, often written mcg/mL. A typical adult value is a single-figure number such as 4.2, which is 4,200 ng/mL
- × 0.0348432
- the molar factor from a nominal mass of 28,700 daltons for the unglycosylated polypeptide. It is offered because readers meet it in conversion tables, not because a laboratory will report it
- why it is only approximate
- circulating IGFBP-3 is glycosylated, and the extent of glycosylation varies, so the apparent mass of the protein in serum ranges from roughly 28 to 45 kilodaltons rather than sitting at one figure. A molar concentration calculated from a single nominal mass can therefore be out by tens of per cent, and no published reference interval on this page is expressed in nmol/L
- which is why laboratories report mass
- every reference interval quoted here, and on the reports these numbers come from, is a mass concentration. Treat the nmol/L column as an illustration of the ladder rather than as a figure to compare against anything
- the ternary complex
- most circulating IGF-1 is carried in a 150 kDa complex of IGF-1, IGFBP-3 and the acid-labile subunit. That is why IGFBP-3 tracks growth hormone status at all — its production is growth-hormone-dependent — and why the IGF-1 to IGFBP-3 ratio is sometimes reported alongside both
Worked example
IGFBP-3 4.2 mg/L in an adult aged 18
4.2 mg/L = 4.2 µg/mL — the same number, often written 4.2 mcg/mL on a report
4.2 × 1000 = 4,200 ng/mL, the headline figure above
4,200 × 0.0348432 = 146.3 nmol/L — an approximation only, because the glycosylated protein does not have a single molecular weight
Against the quoted interval for age 18, 3.1–7.9 mg/L (3,100–7,900 ng/mL), 4.2 mg/L sits within it
Read this alongside an age-matched IGF-1. On its own, IGFBP-3 answers very little
Adult IGFBP-3 reference intervals by age
| Age | mg/L (= µg/mL, mcg/mL) | ng/mL |
|---|---|---|
| 18 | 3.1–7.9 | 3,100–7,900 |
| 19 | 2.9–7.3 | 2,900–7,300 |
| 20 | 2.9–7.2 | 2,900–7,200 |
| 21–25 years | 3.4–7.8 | 3,400–7,800 |
| 26–30 years | 3.5–7.6 | 3,500–7,600 |
| 31–35 years | 3.5–7.0 | 3,500–7,000 |
| 36–40 years | 3.4–6.7 | 3,400–6,700 |
| 41–45 years | 3.3–6.6 | 3,300–6,600 |
| 46–50 years | 3.3–6.7 | 3,300–6,700 |
| 51–55 years | 3.4–6.8 | 3,400–6,800 |
| 56–60 years | 3.4–6.9 | 3,400–6,900 |
| 61–65 years | 3.2–6.6 | 3,200–6,600 |
| 66–70 years | 3.0–6.2 | 3,000–6,200 |
| 71–75 years | 2.8–5.7 | 2,800–5,700 |
| 76–80 years | 2.5–5.1 | 2,500–5,100 |
| 81–85 years | 2.2–4.5 | 2,200–4,500 |
What IGFBP-3 is for, and what it is not for
| Setting | Value of IGFBP-3 | Comment |
|---|---|---|
| Suspected growth hormone deficiency in a child | Useful adjunct to IGF-1 | Less affected by short-term nutritional change than IGF-1, and less likely to be unmeasurably low in young children, which is where IGF-1 is at its weakest |
| Suspected growth hormone deficiency in an adult | Limited | Less sensitive than IGF-1; stimulation testing is what establishes the diagnosis |
| Acromegaly or gigantism | Adds little | IGF-1 is the diagnostic and monitoring measurement; IGFBP-3 contributes almost nothing beyond it |
| Monitoring growth hormone treatment | Occasionally used with IGF-1 | The IGF-1 to IGFBP-3 ratio is reported by some services as a measure of free IGF-1 availability |
| Malnutrition, liver disease, systemic illness | Lowered, like IGF-1 | Does not distinguish growth hormone deficiency from acquired growth hormone resistance, so it does not solve IGF-1’s main weakness |
An adjunct to IGF-1, and an approximate molar figure
Insulin-like growth factor binding protein 3 is the main carrier of IGF-1 in blood. Most circulating IGF-1 travels in a 150-kilodalton ternary complex of IGF-1, IGFBP-3 and the acid-labile subunit, an arrangement that extends IGF-1’s half-life from minutes to hours and keeps it out of the tissues until it is released. Production of IGFBP-3 is itself growth-hormone-dependent, which is the reason it is measured: it tracks growth hormone status, in the same direction as IGF-1 and with a somewhat flatter age curve.
Its clinical place is as an adjunct, and mainly in children. In paediatric growth hormone deficiency the combination of IGF-1 and IGFBP-3 performs better than either measurement alone, partly because IGFBP-3 is less sensitive to short-term changes in nutrition and partly because it remains measurable in young children in whom IGF-1 is often too low to interpret. In adults it is considerably less useful: it is a less sensitive marker of deficiency than IGF-1, and in acromegaly it adds little or nothing to IGF-1, which is the measurement that diagnosis and monitoring rest on. Like IGF-1, it is lowered by malnutrition, liver disease and systemic illness, so it does not distinguish true growth hormone deficiency from acquired resistance.
Two things about the units deserve care. The first is scale. Reports print IGFBP-3 in milligrams per litre, or the numerically identical micrograms per millilitre — often written mcg/mL — so a typical adult result is a single-figure number such as 4.2. In nanograms per millilitre, the unit the molar factor on this site is defined against, the same result is 4,200. A thousandfold slip between those two conventions is the easiest error to make with this analyte.
The second is that the molar conversion is an approximation, and the page says so rather than pretending otherwise. The nmol/L figure here is calculated from a nominal molecular weight of 28,700 daltons, which is the unglycosylated polypeptide. The protein actually circulating is glycosylated to a variable extent, and its apparent mass in serum ranges from roughly 28 to 45 kilodaltons. A molar concentration derived from any single figure is therefore indicative only, and could be out by tens of per cent. This is precisely why laboratories report IGFBP-3 as a mass concentration and why every reference interval on this page is in mass units. Convert to nmol/L if a table asks you to; do not compare the result against anything.
Frequently asked questions
How do I convert IGFBP-3 from mg/L to ng/mL?
Multiply by 1000. Milligrams per litre and micrograms per millilitre are the same number — a report showing 4.2 mcg/mL is 4.2 mg/L and 4,200 ng/mL. That thousandfold step is the commonest transcription error with this analyte, because both conventions are in routine use.
Why is the nmol/L figure for IGFBP-3 only approximate?
Because the circulating protein is glycosylated and the extent varies, so its apparent mass in serum ranges from about 28 to 45 kilodaltons rather than being fixed. The molar factor here uses a nominal 28,700 daltons. Laboratories report IGFBP-3 as a mass concentration and every published reference interval is in mass units, so the molar column is illustrative only.
What is a normal IGFBP-3 level?
It depends on age. On the assay tabulated here it runs from 3.1–7.9 mg/L at eighteen to 2.2–4.5 mg/L at eighty-five, with a plateau in the late teens and early twenties. Intervals differ between assays, so use the one printed on your own report.
Should IGFBP-3 be measured with IGF-1 or instead of it?
With it, and mainly in children. In paediatric growth hormone deficiency the two together perform better than either alone. In adults IGFBP-3 is the less sensitive of the two, and in acromegaly it adds essentially nothing to IGF-1, which is what diagnosis and monitoring rest on.
Does a low IGFBP-3 mean growth hormone deficiency?
Not on its own. IGFBP-3 is lowered by malnutrition, liver disease and systemic illness in the same way IGF-1 is, so it does not separate genuine deficiency from acquired growth hormone resistance. Diagnosis rests on auxology in children and on stimulation testing, with IGF-1 and IGFBP-3 as supporting evidence.
Related calculators
References
- Friedrich N, Wolthers OD, Arafat AM, et al. Age- and sex-specific reference intervals across life span for insulin-like growth factor binding protein 3 (IGFBP-3) and the IGF-I to IGFBP-3 ratio measured by new automated chemiluminescence assays. J Clin Endocrinol Metab. 2014;99(5):1675–1686.
- Mayo Clinic Laboratories. Test IGFB3: Insulin-Like Growth Factor-Binding Protein 3, Serum — age-specific reference values in mcg/mL. Accessed 2026.
- Bidlingmaier M, Friedrich N, Emeny RT, et al. Reference intervals for insulin-like growth factor-1 (IGF-I) from birth to senescence. J Clin Endocrinol Metab. 2014;99(5):1712–1721.
- UniProt Consortium. UniProtKB entry P17936 (IBP3_HUMAN), insulin-like growth factor-binding protein 3. Accessed 2026.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.