IGF-1: Which Tests to Order Alongside for a Complete Clinical Picture

What IGF-1 Actually Measures and Why It Needs Context

Insulin-like growth factor 1 (IGF-1) is a 70-amino-acid peptide produced mainly by hepatocytes in response to growth hormone (GH) binding its receptor. Because GH itself is secreted in pulses (6 to 12 per day, with the largest occurring during slow-wave sleep), a single random GH level is nearly useless for diagnosis [1]. IGF-1, by contrast, has a half-life of 12 to 15 hours and circulates bound to IGF-binding proteins, making it a stable surrogate for integrated daily GH output [2].

But stability does not mean simplicity. IGF-1 concentrations shift with age, sex, nutritional status, liver function, thyroid hormones, and even oral estrogen use. A 35-year-old woman taking combined oral contraceptives can have an IGF-1 level 20 to 40% lower than her untreated baseline without any GH pathology [3]. A malnourished patient with normal GH secretion may show IGF-1 values that mimic GH deficiency. That is why the 2011 Endocrine Society Clinical Practice Guideline states: "A low serum IGF-1 level is suggestive but not proof of GHD; confirmatory GH stimulation testing is required" [4]. Ordering IGF-1 without its companion labs is like reading one chapter of a medical chart and writing a diagnosis.

The Core Paired Tests Every Clinician Should Order

Five labs form the minimum co-panel when IGF-1 is the index test. Each one addresses a known confounder or diagnostic requirement.

IGFBP-3 carries about 80% of circulating IGF-1 in a ternary complex with ALS (acid-labile subunit) [2]. When both IGF-1 and IGFBP-3 are low, the probability of true GH deficiency rises substantially. When IGF-1 is low but IGFBP-3 is normal, consider nutritional or hepatic causes first.

GH stimulation testing is the diagnostic standard. The insulin tolerance test (ITT) remains the Endocrine Society's preferred provocative test, with a GH cutoff of <5 µg/L confirming adult GH deficiency [4]. For patients in whom hypoglycemia is contraindicated (seizure history, ischemic heart disease, age over 65), the glucagon stimulation test serves as an alternative, using the same <3 µg/L cutoff endorsed by the AACE 2019 consensus [5].

Fasting glucose and insulin. GH and IGF-1 directly modulate glucose metabolism. GH promotes lipolysis and opposes insulin action, while IGF-1 has insulin-like glucose-lowering effects. A 2014 meta-analysis in the Journal of Clinical Endocrinology & Metabolism (32 studies, N = 3,817) found that adults with untreated GH deficiency had a 30% higher prevalence of metabolic syndrome compared to age-matched controls [6].

Liver function panel. The liver produces approximately 75 to 80% of circulating IGF-1 [7]. Patients with cirrhosis (Child-Pugh B or C) routinely show IGF-1 levels 50 to 70% below age-matched norms regardless of GH status [7]. Checking ALT, AST, albumin, and bilirubin prevents misattributing hepatic suppression to pituitary failure.

TSH and free T4. Hypothyroidism blunts hepatic IGF-1 synthesis independently of GH secretion. A study of 60 newly diagnosed hypothyroid adults showed mean IGF-1 levels 38% below the age-adjusted median, with normalization after 12 weeks of levothyroxine replacement [8]. If TSH is elevated, treat the thyroid first and recheck IGF-1 before pursuing a GH workup.

Extended Panel: When the Clinical Picture Demands More

Beyond the five core tests, several additional labs become necessary depending on the indication for testing.

Morning cortisol or ACTH stimulation test. Hypopituitarism rarely involves a single axis. The Endocrine Society recommends evaluating all anterior pituitary axes when GH deficiency is suspected in the setting of known pituitary disease [4]. An 8 AM cortisol below 3 µg/dL strongly suggests adrenal insufficiency; values between 3 and 15 µg/dL require dynamic testing with ACTH (cosyntropin) stimulation [9].

Prolactin. Pituitary adenomas that compress the stalk raise prolactin through loss of dopaminergic inhibition ("stalk effect"), and a prolactinoma can coexist with GH co-secretion. Measuring prolactin costs little and can redirect the entire diagnostic path.

HbA1c. In acromegaly workups (where IGF-1 is elevated, not low), glucose tolerance is a treatment outcome measure. The 2014 Endocrine Society acromegaly guideline notes that up to 56% of patients with active acromegaly have impaired glucose tolerance, and 15 to 38% have frank diabetes [10]. HbA1c gives a 90-day glycemic average that complements the fasting glucose.

Sex hormones (total testosterone, estradiol, SHBG). GH therapy in men with concurrent hypogonadism shows blunted IGF-1 response unless testosterone is co-replaced. A 2006 JCEM study (N = 62) demonstrated that GH replacement normalized IGF-1 in eugonadal men within 6 months but required concurrent testosterone therapy to achieve the same result in hypogonadal men [11]. For women on oral estrogen, the first-pass hepatic effect suppresses IGF-1 synthesis, so switching to transdermal estradiol can raise IGF-1 by 20 to 30% without changing GH dose [3].

Complete blood count and ferritin. Iron deficiency can alter hepatic protein synthesis broadly. While not a direct confounder of IGF-1 measurement, severe iron-deficiency anemia signals nutritional compromise that may suppress IGF-1 through caloric or protein insufficiency.

Understanding Normal IGF-1 Ranges by Age

IGF-1 reference ranges are age- and sex-stratified because concentrations vary dramatically across the lifespan. Peak levels occur during puberty (typically 200 to 600 ng/mL between ages 14 and 16), then decline approximately 14% per decade through adulthood [12].

The following approximate adult reference ranges (serum IGF-1, ng/mL) are derived from large population-based assays [12]:

• Ages 21 to 25: 116 to 358 ng/mL
• Ages 26 to 35: 117 to 329 ng/mL
• Ages 36 to 45: 101 to 267 ng/mL
• Ages 46 to 55: 87 to 238 ng/mL
• Ages 56 to 65: 75 to 212 ng/mL
• Ages 66 to 75: 64 to 188 ng/mL
• Ages 76 to 85: 48 to 166 ng/mL

Laboratories use different assay platforms (immunoassay vs. LC-MS/MS), and results are not interchangeable between platforms. The Endocrine Society explicitly recommends using the same assay platform for serial monitoring [4]. A change in lab or assay can produce a 15 to 25% shift in reported values, mimicking a true clinical change when none occurred.

Dr. Beverly Biller, an endocrinologist at Massachusetts General Hospital, has noted: "Clinicians must interpret IGF-1 within the context of the patient's age, sex, nutritional state, and estrogen status. A number without that context is misleading" [4].

How to Raise Low IGF-1 Levels

Low IGF-1 triggers clinical concern, but the correct intervention depends on the cause. Replacing GH in someone whose IGF-1 is low from malnutrition would be inappropriate and potentially harmful.

Treat the underlying cause first. Correct hypothyroidism, restore caloric intake in underweight patients, manage liver disease, and discontinue oral estrogens (or switch to transdermal delivery) before retesting IGF-1. In many cases, the value normalizes without any GH-axis intervention.

GH replacement therapy. For confirmed GH deficiency (failed stimulation test plus low IGF-1), recombinant human GH (somatropin) is the treatment. Starting doses in adults are typically 0.2 to 0.4 mg/day for younger patients and 0.1 to 0.2 mg/day for patients over 60, titrated every 4 to 8 weeks to bring IGF-1 into the upper half of the age-adjusted normal range [4]. The KIMS database (Pfizer International Metabolic Database), which followed over 16,000 GH-deficient adults, showed that maintaining IGF-1 in the 0 to +1 SDS range was associated with the best quality-of-life outcomes without excess adverse events [13].

GH-releasing peptides and secretagogues. Agents like sermorelin, tesamorelin, CJC-1295, and ipamorelin stimulate endogenous GH release rather than replacing GH directly. Tesamorelin is the only FDA-approved GH-releasing hormone analog (approved for HIV-associated lipodystrophy), and it raised IGF-1 by a mean of 81% over baseline in the Phase III trial (N = 412) [14].

Protein and sleep optimization. GH secretion depends on adequate amino acid availability and consolidated slow-wave sleep. Short sleep duration (<6 hours) suppresses nocturnal GH pulses. A controlled study of 11 healthy men showed that restricting sleep to 4 hours for 6 nights reduced the GH pulse area under the curve by 70% [15].

How to Lower Elevated IGF-1 Levels

Elevated IGF-1 most often signals either acromegaly (a GH-secreting pituitary adenoma) or exogenous GH/peptide use. In a longevity context, some clinicians also aim to moderate IGF-1 within the normal range based on epidemiologic data linking higher IGF-1 to certain cancer risks [16].

Acromegaly treatment. The 2014 Endocrine Society guideline recommends transsphenoidal surgery as first-line therapy for most GH-secreting adenomas [10]. Post-surgical biochemical remission (defined as IGF-1 within the age-adjusted normal range) is achieved in 75 to 95% of microadenomas but only 40 to 60% of macroadenomas [10]. For persistent disease, somatostatin receptor ligands (octreotide LAR, lanreotide) reduce IGF-1 to normal in approximately 55% of patients [10]. Pegvisomant, a GH receptor antagonist, normalizes IGF-1 in up to 97% of patients as monotherapy in clinical trials [17].

Dietary and lifestyle modulation. Caloric restriction and intermittent fasting lower IGF-1 in observational and short-term intervention studies. A randomized trial (N = 100) published in Aging Cell found that a 5-day fasting-mimicking diet cycle reduced IGF-1 by 24% from baseline after three monthly cycles [18]. Reducing dairy protein intake may also modestly lower IGF-1, as dairy stimulates hepatic IGF-1 synthesis through both amino acid delivery and direct insulin stimulation [19].

Discontinue exogenous GH or peptides. In patients using GH or secretagogues off-label, IGF-1 elevation resolves within 2 to 4 weeks of cessation due to the peptide's 12 to 15 hour half-life and the downstream normalization of hepatic production.

The 2014 Endocrine Society acromegaly guideline states: "The treatment goal is to normalize IGF-1 for age and sex, as this is associated with restoration of normal mortality rates" [10].

Interpreting IGF-1 in the Context of GH Peptide Therapy

For patients on GH peptide therapy (sermorelin, ipamorelin, CJC-1295/ipamorelin combinations, or tesamorelin), IGF-1 is the primary monitoring biomarker. Checking it alone is still insufficient.

Draw IGF-1 at trough, typically first thing in the morning before the next dose, at least 4 weeks after initiation or dose change. Pair it with fasting glucose and insulin at every monitoring visit because GH-axis stimulation can worsen insulin sensitivity. In the tesamorelin Phase III study, fasting glucose rose by a mean of 3.4 mg/dL at 26 weeks despite the drug's favorable effect on visceral fat [14].

Also recheck thyroid function (TSH, free T4) at 3 and 6 months. GH replacement increases peripheral conversion of T4 to T3, which can unmask central hypothyroidism or increase levothyroxine requirements in patients already on replacement [4]. Missing this interaction leads to persistent fatigue that clinicians may incorrectly attribute to inadequate GH dosing.

IGFBP-3 is worth rechecking at 6 months to confirm that the ternary complex has normalized. If IGF-1 is rising but IGFBP-3 remains flat, consider whether liver function has declined or whether an assay artifact is present.

When to Retest and How Often

The Endocrine Society recommends checking IGF-1 every 4 to 8 weeks during GH dose titration, then every 6 to 12 months once stable [4]. For acromegaly monitoring after surgery, IGF-1 at 12 weeks post-operatively is the first reliable assessment point, as IGF-1 may remain transiently elevated even after successful adenoma resection.

Serial monitoring should use the same laboratory and assay platform. A 2020 study comparing four commercial IGF-1 immunoassays found inter-assay variability of up to 27% for the same sample, meaning that switching laboratories mid-monitoring could falsely suggest treatment failure or overtreatment [20].

Patients receiving GH peptide therapy for longevity or body composition goals should have a minimum panel at each monitoring visit: IGF-1, IGFBP-3, fasting glucose, fasting insulin, HbA1c (every 6 months), and a comprehensive metabolic panel. Adding a lipid panel annually is reasonable given GH's lipolytic effects and its impact on LDL particle size.

The minimum interval between IGF-1 draws after a dose change is 4 weeks; testing earlier reflects the prior dose rather than the new one.