The IGF-1 Standard Deviation Score (IGF-1 SDS), also called the IGF-1 Z-score, expresses an individual's serum IGF-1 (insulin-like growth factor 1) concentration relative to age-matched and sex-matched normative reference data. IGF-1 is a polypeptide hormone produced predominantly in the liver in response to growth hormone (GH) stimulation. Because GH is secreted in episodic pulses throughout the day, a single GH measurement is an unreliable indicator of GH axis status. IGF-1, by contrast, reflects integrated GH secretion over days to weeks, making it the preferred serum biomarker for assessing GH deficiency and GH excess. The SDS is calculated by subtracting the population mean IGF-1 for that age and sex from the individual's measured value, then dividing by the population standard deviation: SDS = (Measured IGF-1 - Mean IGF-1 for age/sex) / SD for age/sex. An IGF-1 SDS above +2 (or above +2.5 in some guidelines) is a key biochemical criterion supporting a diagnosis of GH excess (acromegaly in adults, gigantism in children). An IGF-1 SDS below -2 (or below -1 in some clinical contexts when combined with other evidence) supports GH deficiency. IGF-1 SDS is essential because raw IGF-1 values are meaningless without age-sex normalisation — IGF-1 peaks in puberty (SDS-normalised peak around the time of peak height velocity) and declines substantially through adulthood and old age. A measured IGF-1 of 250 mcg/L may be entirely normal in a 16-year-old but significantly elevated in a 60-year-old.
IGF-1 SDS = (Measured IGF-1 - Mean IGF-1 for age and sex) / SD for age and sex; reference data from assay-specific normative databases (e.g., Bidlingmaier, Elmlinger, or ISPAD tables)
- 1Collect a fasting morning blood sample for serum IGF-1 measurement.
- 2Identify the patient's age (in years, precise) and biological sex for normative table look-up.
- 3Obtain the age-sex-specific mean and standard deviation for IGF-1 from the normative reference database validated for the assay platform used by the laboratory.
- 4Calculate SDS: subtract the population mean from the measured value, then divide by the population SD.
- 5Interpret the SDS in clinical context: SDS >+2 with symptoms of GH excess (acromegaly features, gigantism) supports investigation for GH-secreting tumour; SDS <-2 with symptoms of GH deficiency (fatigue, reduced lean mass, central adiposity, low bone density) supports dynamic testing (insulin tolerance test or GHRH-arginine stimulation test).
- 6Note that IGF-1 SDS alone is not diagnostic — multiple measurements and clinical correlation are required.
- 7Ensure the normative reference data used are assay-specific — IGF-1 values differ substantially between different immunoassay platforms and cannot be compared without cross-platform validation.
Strongly supports GH excess — oral glucose suppression test and pituitary MRI required
An SDS of +8.6 is far above the +2 threshold and is highly specific for GH excess. In the context of clinical features (enlarged hands and feet, prognathism, coarsened features, sweating, sleep apnoea), this result is diagnostic of acromegaly pending confirmatory glucose suppression testing.
High absolute IGF-1 is normal in puberty — SDS interpretation prevents misdiagnosis
An absolute IGF-1 of 680 mcg/L sounds high, but in a mid-pubertal boy this is within the normal SDS range (+0.56). Without age-sex normalisation, this value might be mistakenly flagged as elevated. SDS normalisation is critical in adolescents.
Borderline — dynamic GH stimulation testing recommended
An SDS of -2.0 in a patient with pituitary disease and symptoms of GH deficiency (fatigue, central adiposity, poor quality of life) warrants formal dynamic GH testing. Most guidelines recommend an insulin tolerance test (or GHRH-arginine if ITT contraindicated) to confirm GH deficiency in this context.
Low absolute IGF-1 is physiological in elderly — SDS is normal
IGF-1 declines progressively after age 30. An absolute value of 68 mcg/L would be clearly low in a 30-year-old but is within the normal SDS range for a 72-year-old. Age-adjusted interpretation prevents over-investigation in the elderly.
Professionals in finance and lending use Igf1 Sds as part of their standard analytical workflow to verify calculations, reduce arithmetic errors, and produce consistent results that can be documented, audited, and shared with colleagues, clients, or regulatory bodies for compliance purposes.
University professors and instructors incorporate Igf1 Sds into course materials, homework assignments, and exam preparation resources, allowing students to check manual calculations, build intuition about input-output relationships, and focus on conceptual understanding rather than arithmetic.
Consultants and advisors use Igf1 Sds to quickly model different scenarios during client meetings, enabling real-time exploration of what-if questions that would otherwise require returning to the office for detailed spreadsheet-based analysis and reporting.
Individual users rely on Igf1 Sds for personal planning decisions — comparing options, verifying quotes received from service providers, checking third-party calculations, and building confidence that the numbers behind an important decision have been computed correctly and consistently.
Extreme input values
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in igf1 sds calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Assumption violations
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in igf1 sds calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Rounding and precision effects
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in igf1 sds calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
| IGF-1 SDS | Interpretation | Clinical Action |
|---|---|---|
| > +2.5 | Markedly elevated — GH excess likely | Oral glucose suppression test + pituitary MRI |
| +2.0 to +2.5 | Elevated — investigate | Glucose suppression test; clinical assessment |
| -1.0 to +2.0 | Normal range | No action if asymptomatic |
| -2.0 to -1.0 | Low-normal — monitor | Clinical assessment; consider dynamic testing if symptomatic |
| < -2.0 | Low — GH deficiency possible | Dynamic GH stimulation test required |
| < -3.0 | Markedly low | Strong support for GH deficiency; dynamic testing essential |
Why is IGF-1 preferred over GH for assessing the GH axis?
GH is secreted in episodic pulses throughout the day and has a short half-life of approximately 20 minutes. A single random GH measurement can be undetectably low in a healthy person (between pulses) or elevated due to physiological stimuli (exercise, stress, sleep). IGF-1, produced by the liver in response to sustained GH exposure, has a half-life of approximately 15 hours and is far more stable, reflecting integrated GH activity over days.
What conditions cause a falsely low IGF-1 SDS?
IGF-1 production depends not just on GH but also on nutritional status, hepatic function, and insulin availability. IGF-1 is falsely low (relative to GH axis function) in malnutrition, anorexia nervosa, hepatic failure, uncontrolled diabetes, hypothyroidism, and inflammatory states. In these conditions, a low IGF-1 SDS may not reflect true GH deficiency.
What conditions cause a falsely high IGF-1 SDS?
In the context of Igf1 Sds, this depends on the specific inputs, assumptions, and goals of the user. The underlying formula provides a deterministic relationship between inputs and output, but real-world application requires interpreting the result within the broader context of finance and lending practice. Professionals typically cross-reference calculator output with industry benchmarks, historical data, and regulatory requirements. For the most reliable results, ensure inputs are sourced from verified data, understand which assumptions the formula makes, and consider running multiple scenarios to bracket the range of likely outcomes.
How is acromegaly confirmed after an elevated IGF-1 SDS?
Acromegaly is confirmed by failure of GH to suppress below 1 ng/mL (0.4 mcg/L with sensitive assays) during an oral glucose tolerance test (75g glucose, GH at 0, 30, 60, 90, 120 minutes). The combination of elevated IGF-1 SDS and non-suppressed GH on OGTT is diagnostic. Pituitary MRI identifies the causative adenoma in most cases.
Does IGF-1 SDS change with GH therapy?
Yes. IGF-1 SDS is used to monitor and titrate GH replacement therapy in GH-deficient adults and children. Most guidelines recommend targeting an IGF-1 SDS between 0 and +2 during GH replacement — sufficient to normalise metabolic parameters without driving IGF-1 to supranormal levels that may be associated with increased cancer risk in some studies.
Are there different normative reference databases for IGF-1?
In the context of Igf1 Sds, this depends on the specific inputs, assumptions, and goals of the user. The underlying formula provides a deterministic relationship between inputs and output, but real-world application requires interpreting the result within the broader context of finance and lending practice. Professionals typically cross-reference calculator output with industry benchmarks, historical data, and regulatory requirements. For the most reliable results, ensure inputs are sourced from verified data, understand which assumptions the formula makes, and consider running multiple scenarios to bracket the range of likely outcomes.
What IGF-1 SDS threshold is used to diagnose acromegaly?
An IGF-1 SDS above +2 is the most widely used threshold for biochemical suspicion of GH excess. Some guidelines (e.g., Endocrine Society 2014) note that an SDS above +2.5 or above the laboratory's age-sex-specific upper limit of normal should prompt confirmatory glucose suppression testing. The exact threshold varies by assay and guideline.
Is IGF-1 SDS used in childhood GH deficiency?
Yes. In children, an IGF-1 SDS below -2 in the context of poor growth velocity and other features of GH deficiency (growth velocity SDS below -1, bone age delay, auxological criteria) is supportive but not diagnostic of GH deficiency. Confirmatory GH stimulation tests (arginine, glucagon, GHRH-arginine) are required before GH therapy is initiated.
Pro Tip
Always ask the laboratory which normative reference database they use for IGF-1 SDS calculation and which assay platform measures the IGF-1. If a patient moves between hospitals that use different assay platforms, the two IGF-1 values may not be directly comparable even if reported in the same units.
Did you know?
IGF-1 was originally named 'somatomedin C' — meaning 'a substance that mediates the effects of somatotropin (growth hormone)' — before its full structure was characterised and it was renamed for its structural similarity to proinsulin. The two hormones share approximately 50% structural similarity, which explains why supraphysiological IGF-1 can activate insulin receptors and cause hypoglycaemia.
References
- ›Katznelson L et al. Acromegaly: An Endocrine Society Clinical Practice Guideline 2014
- ›Molitch ME et al. Evaluation and Treatment of Adult GH Deficiency (Endocrine Society 2011)
- ›Bidlingmaier M et al. Reference ranges for insulin-like growth factor-1 (IGF-1) from birth to senescence. Eur J Endocrinol 2014
- ›NICE TA188 — Human growth hormone in adults with GH deficiency 2003 (reviewed 2012)