How to prepare for an IGF-1 blood test: the rules of sampling
Insulin-like growth factor 1 (IGF-1) is one of the most informative markers of the growth-hormone axis, and clinicians in the UK use it to investigate suspected acromegaly, growth-hormone deficiency, and a range of growth disorders. Unlike growth hormone itself, IGF-1 stays relatively stable through the day, which makes it a reliable single-sample test — but only if the sample is taken correctly. The editorial team has gathered the practical rules that help you avoid a result that has to be repeated.
What IGF-1 actually measures
IGF-1 is a small protein produced mainly by the liver in response to growth hormone (GH). Because GH is released in short bursts and swings widely within minutes, a single GH measurement tells you very little. IGF-1, by contrast, reflects the average GH output over the previous day or two, so a single blood draw gives a far more stable picture of the axis.
In adults, the test is ordered most often when a doctor suspects acromegaly — a condition of GH excess — or, at the other end, adult GH deficiency. In children and adolescents it helps investigate short stature or unusually rapid growth. In the UK these tests are requested through the NHS and interpreted against age- and sex-specific reference ranges, because IGF-1 falls naturally with age.
It is worth being clear about what the test is not. IGF-1 is not a routine fitness or anti-ageing marker, and a single value in isolation rarely diagnoses anything. Laboratories and endocrinologists read it alongside clinical signs and, where needed, dynamic tests such as the oral glucose tolerance test or GH stimulation testing.
Finally, assays differ between laboratories. A value from one lab is not always directly comparable with another, which is why clinicians prefer serial measurements from the same service. If you are tracking a result over time, try to use the same laboratory each time.
Timing, fasting and the day before
The good news is that IGF-1 does not require the strict timing that GH does. It can usually be sampled at any time of day, and most UK laboratories do not insist on an overnight fast purely for IGF-1. However, if the blood test is bundled with other analytes — glucose, lipids or an insulin measurement — the fasting rules of those tests take over, and you should follow the stricter instruction.
Several everyday factors nudge IGF-1 up or down and are worth managing in the days before. Prolonged fasting, very low-calorie dieting and poorly controlled diabetes tend to lower IGF-1, because the liver's response to GH depends on adequate nutrition and insulin. Severe illness and liver disease also depress it. Conversely, nothing you eat on the morning of the test will push a normal result into the abnormal range.
Keep your routine ordinary. A crash diet or an unusually heavy training session in the preceding days can shift the picture, so it is sensible to test during a stable period rather than at the end of a hard cut or a bout of illness.
Always tell the requesting clinician about any hormone treatment. Oestrogens taken by mouth, growth-hormone therapy and some other medicines change IGF-1 meaningfully. This is not something to adjust yourself; it is information the laboratory and doctor need in order to read the number correctly.
On the day: the practical checklist
Preparation on the day is mostly about making the blood draw itself clean and the result interpretable. The list below covers the points that most often cause a sample to be rejected or a value to be questioned.
- Hydration: drink water normally; being well hydrated makes venepuncture easier and does not affect IGF-1.
- Biotin supplements: high-dose biotin can interfere with some immunoassays. Many labs advise pausing it for a couple of days — check with the requesting service.
- Medication list: bring an up-to-date list, including supplements and any hormone products, and share it with the phlebotomist or clinician.
- Recent illness: if you are acutely unwell, ask whether the test should be postponed.
- Same laboratory: for repeat testing, use the same service to keep assays comparable.
During the draw a standard venous sample is taken from the arm, usually into a serum or plasma tube according to local protocol. There is nothing special you need to do beyond sitting still and relaxed; the stress of a difficult draw does not materially change IGF-1 the way it can change some stress hormones.
After the appointment, apply gentle pressure to the puncture site and keep the result for your records. If the value is flagged, resist the urge to interpret it alone — the next section explains why context matters so much.
Reading the result in context
An IGF-1 result arrives as a number with a reference range matched to your age and sex. Because the range narrows and shifts across the lifespan, the same figure can be normal at twenty and abnormal at sixty, or vice versa. This is why you should never compare your value with someone else's without accounting for age.
A high IGF-1 raises the question of GH excess and, in the UK, usually prompts confirmatory testing — classically an oral glucose tolerance test with GH measurement, since glucose should suppress GH in healthy people but fails to in acromegaly. A low value may point towards GH deficiency, but it can equally reflect malnutrition, liver disease or poorly controlled diabetes, so the clinical picture decides.
The table below summarises common factors that move IGF-1 and the direction of their effect. It is a general guide, not a diagnostic tool.
| Factor | Typical effect on IGF-1 | Why it matters for your test |
|---|---|---|
| Age | Falls with age | Results read against age-specific ranges |
| Prolonged fasting / crash diet | Lowers | Test during a stable period |
| Poorly controlled diabetes | Lowers | Mention to clinician |
| Oral oestrogen therapy | Lowers | Declare all hormone use |
| Growth-hormone therapy | Raises | Declare; alters interpretation |
| Liver disease | Lowers | Part of clinical context |
In short, preparation is less about elaborate rules and more about testing when you are well-fed, well, and honest about your medication. The laboratory does the rest.
Common questions and mistakes
One recurring question is whether exercise before the test matters. A single ordinary workout has little effect on IGF-1, but a sustained period of overtraining combined with under-eating can lower it, so avoid testing at the tail end of an aggressive cut. Another frequent query concerns supplements marketed as "GH boosters"; most have no proven effect on measured IGF-1, and none should be relied upon to change a clinical result.
A common mistake is self-ordering the test through a private panel, getting a borderline figure, and panicking. Borderline IGF-1 values are common and usually benign. The right response is to discuss the number with a clinician who can see the whole picture, not to repeat the test obsessively or start interventions on your own.
People also worry about biotin, and rightly so in some cases. High-dose biotin supplements, popular for hair and nails, can distort certain immunoassays in either direction. If you take them, pausing for a day or two before sampling — on your lab's advice — removes the uncertainty.
Readers interested in this area may also find our material on preparing for a fasting glucose test, on interpreting a thyroid panel, and on the basics of hormonal monitoring useful as companion reading.
Conclusions of the editorial team
IGF-1 is a stable, informative marker of the growth-hormone axis, and for most people preparing for it is straightforward: test when you are well and well-nourished, keep your routine ordinary, and declare every medicine and supplement you take.
The value only means something against an age- and sex-specific range and in the context of your symptoms, so avoid interpreting a single figure alone. Use the same laboratory for repeat tests to keep results comparable.
If a result is flagged, the next step is a conversation with your GP or an endocrinologist, not a self-directed intervention. Good preparation simply ensures the number they see is a true one.
References
- Clemmons DR. Consensus statement on the standardization and evaluation of growth hormone and insulin-like growth factor assays. Clin Chem. 2011;57(4):555-559.
- Katznelson L, Laws ER Jr, Melmed S, et al. Acromegaly: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2014;99(11):3933-3951.
- Molitch ME, Clemmons DR, Malozowski S, et al. Evaluation and treatment of adult growth hormone deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(6):1587-1609.
- Juul A. Serum levels of insulin-like growth factor I and its binding proteins in health and disease. Growth Horm IGF Res. 2003;13(4):113-170.
- Li D, Radulescu A, Shrestha RT, et al. Association of biotin ingestion with performance of hormone and nonhormone assays. JAMA. 2017;318(12):1150-1160.
- National Institute for Health and Care Excellence. Acromegaly: diagnosis and management. NICE guidance. London: NICE.
