Four small blood-sample tubes with different coloured caps — sage green, gold, lilac and blue — standing in a warm oak rack on a sunlit windowsill beside a leafy plant, calm and domestic rather than clinical.

Growth science · Medical tests

Your child's growth blood tests, explained

GrowSense Growth Science · Educational, not medical advice

Every claim sourced to peer-reviewed research — see references below

Your child is growing slowly, and the doctor orders bloods. A few days later you're holding a page of numbers — IGF-1, TSH, free T4, vitamin D, ferritin — each stamped low, normal or high. And the label answers everything except the one thing you want to know: does this explain why my child isn't growing?

Usually, no single number can. Growth is produced by a connected system — the brain signals, the thyroid sets the pace, nutrition supplies materials, iron carries oxygen, vitamin D hardens new bone. A blood test is one clue inside that system, not a height prediction. This guide digests what each of the four common results means, and — the part that matters most — how they fit together.

First, the reframe that prevents most panic. Blood tests do not measure growth. They investigate reasons for an abnormal growth pattern. The measurements that actually track growth are still height, growth velocity over time, position on the right chart, pubertal stage, family height, and bone age when needed.

A child can grow slowly with normal bloods, and have a mildly "abnormal" result that has nothing to do with height. That's why a doctor reads the numbers with the growth curve, not instead of it.

Ranges vary — a lot. Reference ranges shift with age, sex, puberty, the testing method, time of day, recent illness, body composition and the lab's own population. Always read a result against the range printed by that laboratory. A value just outside one lab's range can sit inside another's.

1. IGF-1 — the messenger from the growth-hormone system

Growth hormone (GH) is released from the pituitary in pulses, mostly during deep sleep. Because it spikes and crashes through the day, a single random GH blood test is nearly useless. So doctors measure IGF-1 instead — insulin-like growth factor 1, made by the liver in response to GH. It stays steady for hours, which makes it a practical window onto the whole GH system.

The number alone means little; the age-adjusted version means a lot. “IGF-1: 125” is meaningless without context — normal for a young child, low for a teenager. What clinicians actually read is the IGF-1 SDS (standard-deviation score, or z-score): how far the result sits from the average for a child of that age and sex. Near 0 is average; around −2 is near the low edge of normal.

A low IGF-1 is a flag, not a verdict. It can point toward growth-hormone deficiency — but it is not specific to it. It also drops with too little food or protein, coeliac disease and malabsorption, chronic inflammation, liver or kidney disease, hypothyroidism, delayed puberty, and simply being very young.

The finding that surprises parents. IGF-1 is a genuinely poor stand-alone test for GH deficiency. Some children with confirmed deficiency have IGF-1 inside the normal range; many children with a low IGF-1 have no deficiency at all.[2][3] It earns its place as one piece of a combined picture, never as a pass/fail.
Parent translation. A low IGF-1 means the growth-hormone pathway may be underactive — or the body simply may not have the nutrition, thyroid hormone or general health to produce a normal level. It does not, by itself, mean your child needs growth hormone.[1][4]

A normal IGF-1 makes severe GH deficiency less likely — especially if growth velocity is normal — but doesn’t fully rule it out. When a child is growing very slowly, crossing downward, or has pituitary risk factors, doctors may go further with GH stimulation testing. Even that has limits: a higher BMI blunts the GH response, so a heavier child can look deficient without truly being so.[5]

The mechanism is covered in Why do some children grow faster than others? — GH and IGF-1 act together at the growth plate, where cartilage cells multiply and are replaced by bone.

2. TSH and free T4 — is the thyroid keeping pace?

These two are a pair, and reading either one alone is where mistakes happen.

TSH (thyroid-stimulating hormone) is made by the pituitary — it’s the brain’s request for thyroid hormone. Free T4 is the actual hormone the thyroid releases into the blood. The cleanest way to hold them together:

Thermostat (TSH) asks for heat → Furnace (thyroid) produces heat → Free T4 is the heat delivered. If free T4 drops, the thermostat turns up — TSH rises to demand more.

The patterns that matter:

Why the range seems to move. TSH follows a daily rhythm and rises overnight; it also shifts with age and method.[10] A result just outside one lab's adult range can be squarely normal on a paediatric one. This is the test where "slightly abnormal" most often means "recheck, don't react."

Without enough thyroid hormone, the growth plate can’t mature its cells in the right sequence, bone age falls behind, and IGF-1 often drops too — which is why hypothyroidism can produce low growth, delayed bone age and low IGF-1 all at once. Treating the thyroid can restore several of those at the same time.

3. Vitamin D — a mineralisation test, not a height score

Order the right one. The test that reflects your child’s stores is 25-hydroxyvitamin D (25(OH)D). Don’t confuse it with the active hormone (1,25-dihydroxyvitamin D), which can stay normal — or even rise — during ordinary deficiency, and is the wrong test for stores.

Mind the units. 1 ng/mL = 2.5 nmol/L. So 20 ng/mL = 50 nmol/L, and 30 ng/mL = 75 nmol/L. There’s no single universal cutoff, but broadly: below ~12 ng/mL (30 nmol/L) is clearly deficient; 12–20 (30–50) is low/insufficient in many systems; ≥20 ng/mL (50 nmol/L) is adequate for most healthy children.[15][16]

A concrete anchor. In one paediatric study, parathyroid hormone started climbing once 25(OH)D fell below about 34 nmol/L, and every child with X-ray-confirmed rickets was below that level — but the authors stressed that calcium intake has to be judged alongside it, not vitamin D in isolation.[11]

Low vitamin D means less support for absorbing calcium and phosphate — the minerals that harden newly built bone. Severe, prolonged deficiency can cause nutritional rickets, where the growth plate can’t mineralise and you may see bowed legs, widened wrists, bone pain, and a raised alkaline phosphatase and PTH.

The myth to retire. Correcting real deficiency matters for bone. But extra vitamin D above adequate has not been shown to make healthy children taller. A three-year trial in vitamin-D-deficient schoolchildren improved their vitamin D status substantially yet did not change height, body composition or puberty;[13] systematic reviews reach the same conclusion.[12][14] Vitamin D hardens the bone being built — it isn't an accelerator that pushes height past the child's programme.

Because of this, a 25(OH)D number is rarely read alone. When deficiency looks clinically important, doctors add calcium, phosphate, alkaline phosphatase, PTH and sometimes an X-ray — the number shows stores, not whether the growth plate is actually mineralising. The sunlight side of this is covered in Growth supplements that matter.

4. Ferritin — the body’s iron store

Ferritin is not haemoglobin. Picture it as three separate things: ferritin is the storeroom (how much iron is banked), transferrin is the delivery van, and haemoglobin is the oxygen-carrying equipment. A child can empty the storeroom — low ferritin — before haemoglobin falls far enough to count as anaemia. That makes ferritin an early warning.

A genuinely low ferritin is strong evidence of depleted iron stores — from low intake, rapid growth, heavy periods in teens, gut blood loss, or malabsorption like coeliac disease. Cutoffs vary by age and lab, so clinicians pair it with the blood count (haemoglobin, MCV, MCH) and iron studies rather than one online figure.[18]

The trap: a "normal" ferritin can be lying. Ferritin is an acute-phase reactant — it rises with infection, inflammation, obesity or recent illness. So a child can be iron-deficient while ferritin reads normal or even high, because inflammation is pushing it up. The fix is to read it alongside an inflammation marker like CRP.[17] Low ferritin is usually meaningful; a normal ferritin only reassures when the child isn't inflamed.

Can low iron slow growth? Severe or prolonged iron-deficiency anaemia can go with reduced growth velocity, poor appetite and fatigue — one study found young children with IDA were shorter, with slower velocity, and improved after treatment.[19] But iron is not a height supplement. Meta-analyses find no consistent growth benefit from giving iron broadly to children who aren’t clearly deficient, and excess iron carries its own risks.[20][21][22] Iron corrects a demonstrated need — it isn’t a grow-taller pill.

5. How the four connect — the part a single number hides

This is the heart of it. The results arrive in separate boxes, but the biology is one circuit, and the pattern tells the story.[23][24]

Thyroid enables the GH–IGF-1 system. Adequate thyroid hormone is needed for normal GH release, liver IGF-1 production and growth-plate maturation. So a hypothyroid child can show both poor growth and a low IGF-1 — and treating the IGF-1 without spotting the thyroid would miss the real, upstream cause.

Nutrition decides whether IGF-1 can even rise. GH can’t build growth from nothing. When energy or protein is short, the liver becomes relatively resistant to GH — GH may be normal or high while IGF-1 stays low, a deliberate braking of expensive growth during scarcity. Chronic inflammation causes the same GH resistance.[25] This is why a low IGF-1 is not a synonym for pituitary failure.

Vitamin D and calcium harden what the growth plate builds. The plate lays down a cartilage scaffold; minerals turn it into bone. In plain terms: GH and IGF-1 build the scaffold, thyroid hormone sequences the cells, vitamin D and calcium harden the structure.

Iron powers the whole factory. Growth-plate cells are metabolically busy — iron drives oxygen transport, cellular energy, and even thyroid-hormone metabolism. Severe deficiency starves the process from several directions at once.

And inflammation can disturb several tests together. Chronic inflammation can suppress appetite, lower IGF-1, cause GH resistance, and push ferritin up despite unavailable iron.[26]

Read the pattern, not the box. A child with low IGF-1 + normal-to-high ferritin + anaemia + slow growth is far more likely telling a story of inflammation than of abundant iron. Each number alone misleads; together they point one way.

6. Reading the pattern

Result patternMay suggestDoes not prove
Low IGF-1 aloneGH issue, undernutrition, delayed puberty, or chronic diseaseThat GH treatment is needed
Low IGF-1 + low free T4 + high TSHPrimary hypothyroidism affecting the growth axisIsolated GH deficiency
Low IGF-1 + low weight / poor intakeNutritional suppression or malabsorptionPituitary disease
High TSH + normal free T4Mild/subclinical thyroid change, or obesity effectA definite cause of short stature
Low free T4 + normal/low TSHPossible central hypothyroidism or illness effectNormal thyroid function
Low vitamin D onlyReduced vitamin D storesThe main cause of slow growth
Very low vitamin D + abnormal calcium/phosphate/ALPDisturbed mineralisation, possible ricketsA simple dietary issue only
Low ferritin + low MCV/MCHIron deficiency, possibly progressing to anaemiaThat iron is the only growth problem
Normal-high ferritin + raised CRPInflammation may be hiding iron deficiencyStrong iron stores
All four normalThese pathways look reassuringThat every cause of poor growth is excluded
What these tests can't do. Even a fully normal panel can't predict adult height, read bone age, prove GH deficiency, show whether growth plates are open, or exclude coeliac, kidney, inflammatory, skeletal or genetic causes.[27] A normal screen is useful — but it doesn't end the investigation when growth velocity is clearly abnormal.

7. Making the appointment count

The most valuable thing you can bring is good questions:

Was the IGF-1 interpreted for my child's age, sex and puberty — and shown as an SDS or z-score?

This is the single most important thing about an IGF-1 result. A raw number without age adjustment can't be judged; the SDS is what tells you where your child actually sits.

Could nutrition, illness or thyroid explain a low IGF-1?

Often yes. A low IGF-1 is a shared endpoint of several very different situations,[1] so the useful question is which upstream cause fits the rest of the picture.

Were TSH and free T4 read together?

They have to be. TSH alone can miss central hypothyroidism (low free T4 without the expected TSH rise),[6] and a mildly high TSH with normal free T4 usually means "recheck," not "treat."

Was my child ill when ferritin was measured, and was CRP checked?

Because ferritin rises with inflammation, a "normal" value during or just after illness can hide real iron deficiency.[17] CRP tells you whether to trust the ferritin.

Does the vitamin D result need calcium, phosphate, ALP or PTH for context?

A 25(OH)D number shows stores, not whether bone is mineralising. When it's low enough to matter, those companion tests are what show the effect on the skeleton.[11]

Is my child's growth velocity actually abnormal — and would rechecking beat acting on one borderline value?

Velocity over time is the real signal. A single borderline number is often best repeated rather than acted on, especially if the child is growing steadily along their own curve.

8. When a result deserves prompt review

Seek timely medical follow-up when blood results come with any of: markedly slow growth velocity or falling across percentile bands; headaches or visual symptoms; excessive thirst or urination; persistent vomiting or diarrhoea; unusually early or delayed puberty; significant fatigue, pallor or breathlessness; bone pain or limb deformity; an enlarged thyroid; a very low free T4, substantially high TSH, severe anaemia, or very low vitamin D with abnormal minerals. It’s the combination of an abnormal number and an abnormal child that matters — not the number by itself.

How this connects to the whole system

Every one of these tests is a single instrument reading one part of a connected machine. That’s the same reason a bone age, a height percentile or a food log means little alone — the signal is always in the relationship between them, followed over time. Growth is an emergent property of the whole system, and catch-up growth after a cause is corrected is one of the clearest demonstrations of that.[25]

Your child's labs, kept as a story — not a scatter of numbers

GrowSense holds each result next to the things that give it meaning: height velocity, weight, pubertal signs and previous labs, on one honest timeline — with a plain-language read of what the pattern is saying and what would sharpen it. Not to replace your doctor, but so you walk into the appointment understanding the numbers instead of fearing them.

Explore GrowSense

The parent takeaway

No growth blood test means anything on its own. IGF-1 is a steady window on the growth-hormone system, but it drops for nutrition, thyroid and illness too. TSH and free T4 must be read as a pair. Vitamin D reflects bone mineralisation, not height. Ferritin estimates iron stores — trustworthy when low, potentially misleading when the child is inflamed.

So the question that actually helps your child is never “is this one number normal?”

It’s “does the whole pattern — velocity, weight, puberty, bone age, symptoms and labs — tell a coherent story?”

A single result is a data point. The pattern is the diagnosis.

References

IGF-1 and the growth-hormone system

  1. Ibba A, Corrias F, Guzzetti C, Casula L, et al. IGF1 for the diagnosis of growth hormone deficiency in children and adolescents: a reappraisal. Endocr Connect. 2020;9(11):1095–1102. PMID: 33112822.
  2. Fatani TH. Diagnostic Value of IGF-1 in Growth Hormone-Deficient Children: Is a Second Growth Hormone Stimulation Test Necessary? J Endocr Soc. 2023;7(4):bvad018. PMID: 36846213.
  3. Iwayama H, Kitagawa S, Sada J, Miyamoto R, et al. Insulin-like growth factor-1 level is a poor diagnostic indicator of growth hormone deficiency. Sci Rep. 2021;11(1):16159. PMID: 34373538.
  4. Yuen KCJ, Johannsson G, Ho KKY, Miller BS, et al. Diagnosis and testing for growth hormone deficiency across the ages: a global view of the accuracy, caveats, and cut-offs for diagnosis. Endocr Connect. 2023;12(7):e230023. PMID: 37052176.
  5. Stanley TL, Levitsky LL, Grinspoon SK, Misra M. Effect of body mass index on peak growth hormone response to provocative testing in children with short stature. J Clin Endocrinol Metab. 2009;94(12):4875–4881. PMID: 19890023.

TSH, free T4 and the thyroid

  1. Leung AKC, Leung AAC. Evaluation and management of the child with hypothyroidism. World J Pediatr. 2019;15(2):124–134. PMID: 30734891.
  2. Rivkees SA, Bode HH, Crawford JD. Long-term growth in juvenile acquired hypothyroidism: the failure to achieve normal adult stature. N Engl J Med. 1988;318(10):599–602. PMID: 3344006.
  3. Becker M, Blankenstein O, Lankes E, Schnabel D, et al. Severe Acquired Primary Hypothyroidism in Children and its Influence on Growth: A Retrospective Analysis of 43 Cases. Exp Clin Endocrinol Diabetes. 2022;130(4):217–222. PMID: 34607373.
  4. Gyuricsko E. The "slightly" abnormal thyroid test: What is the pediatrician to do? Curr Probl Pediatr Adolesc Health Care. 2020;50(3):100770. PMID: 32418870.
  5. Ehrenkranz J, Bach PR, Snow GL, Schneider A, et al. Circadian and Circannual Rhythms in Thyroid Hormones: Determining the TSH and Free T4 Reference Intervals Based Upon Time of Day, Age, and Sex. Thyroid. 2015;25(8):954–961. PMID: 26061389.

Vitamin D, mineralisation and growth

  1. Atapattu N, Shaw N, Högler W. Relationship between serum 25-hydroxyvitamin D and parathyroid hormone in the search for a biochemical definition of vitamin D deficiency in children. Pediatr Res. 2013;74(5):552–556. PMID: 23999068.
  2. Huey SL, Acharya N, Silver A, Sheni R, et al. Effects of oral vitamin D supplementation on linear growth and other health outcomes among children under five years of age. Cochrane Database Syst Rev. 2020;12(12):CD012875. PMID: 33305842.
  3. Ganmaa D, Bromage S, Khudyakov P, Erdenenbaatar S, et al. Influence of Vitamin D Supplementation on Growth, Body Composition, and Pubertal Development Among School-aged Children in an Area With a High Prevalence of Vitamin D Deficiency: A Randomized Clinical Trial. JAMA Pediatr. 2023;177(1):32–41. PMID: 36441522.
  4. Ma K, Wei SQ, Bi WG, Weiler HA, et al. Effect of Vitamin D Supplementation in Early Life on Children's Growth and Body Composition: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Nutrients. 2021;13(2):524. PMID: 33562750.
  5. Holick MF, Binkley NC, Bischoff-Ferrari HA, Gordon CM, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911–1930. PMID: 21646368.
  6. Demay MB, Pittas AG, Bikle DD, Diab DL, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024;109(8):1907–1947. PMID: 38828931.

Ferritin and iron

  1. Namaste SM, Rohner F, Huang J, Bhushan NL, et al. Adjusting ferritin concentrations for inflammation: Biomarkers Reflecting Inflammation and Nutritional Determinants of Anemia (BRINDA) project. Am J Clin Nutr. 2017;106(Suppl 1):359S–371S. PMID: 28615259.
  2. Moscheo C, Licciardello M, Samperi P, La Spina M, et al. New Insights into Iron Deficiency Anemia in Children: A Practical Review. Metabolites. 2022;12(4):289. PMID: 35448476.
  3. Soliman AT, Al Dabbagh MM, Habboub AH, Adel A, et al. Linear growth in children with iron deficiency anemia before and after treatment. J Trop Pediatr. 2009;55(5):324–327. PMID: 19261662.
  4. Sachdev H, Gera T, Nestel P. Effect of iron supplementation on physical growth in children: systematic review of randomised controlled trials. Public Health Nutr. 2006;9(7):904–920. PMID: 17010257.
  5. Thompson J, Biggs BA, Pasricha SR. Effects of daily iron supplementation in 2- to 5-year-old children: systematic review and meta-analysis. Pediatrics. 2013;131(4):739–753. PMID: 23478873.
  6. Lönnerdal B. Excess iron intake as a factor in growth, infections, and development of infants and young children. Am J Clin Nutr. 2017;106(Suppl 6):1681S–1687S. PMID: 29070544.

How the pathways connect

  1. Yakar S, Isaksson O. Regulation of skeletal growth and mineral acquisition by the GH/IGF-1 axis: Lessons from mouse models. Growth Horm IGF Res. 2016;28:26–42. PMID: 26432542.
  2. Bogarín R, Richmond E, Rogol AD. A new approach to the diagnosis of short stature. Minerva Pediatr. 2020;72(4):250–262. PMID: 33045802.
  3. Ranke MB. Catch-up growth: new lessons for the clinician. J Pediatr Endocrinol Metab. 2002;15 Suppl 5:1257–1266. PMID: 12510976.
  4. Yakoob MY, Lo CW. Nutrition (Micronutrients) in Child Growth and Development: A Systematic Review on Current Evidence, Recommendations and Opportunities for Further Research. J Dev Behav Pediatr. 2017;38(8):665–679. PMID: 28746059.
  5. Wit JM, Kiess W, Mullis P. Genetic evaluation of short stature. Best Pract Res Clin Endocrinol Metab. 2011;25(1):1–17. PMID: 21396571.
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This article is educational and does not diagnose any condition or interpret your child's specific results. Blood tests must be read by a qualified clinician using the testing laboratory's own reference ranges and your child's full clinical picture. If your child's growth is a concern, or a result is flagged abnormal, discuss it with your paediatrician or paediatric endocrinologist rather than acting on a single number.