Radioactive Iodine Uptake (RAIU) Interpreter

RAIU is already a percentage, so there is nothing to convert. What there is to do with it is the question the test was invented to answer: in a thyrotoxic patient, is the gland making too much hormone or leaking hormone it already made? High uptake and low uptake separate those two, and they are treated in opposite ways.

Last updated 18 Sep 2026

RAIU interpretation

Uptake % → differential

24-hour uptake (%)The standard measurement. Normal ranges depend on local iodine intake and must come from the laboratory that made the measurement — the figures this page uses are given in the table below with their dates and countries attached.

4–6 hour uptake (%)Optional, and useful mainly for one thing: a small group of Graves’ patients turn iodine over so fast that their peak uptake is at 6 to 12 hours and the 24-hour figure has already fallen from it. An early value higher than the 24-hour value is the signature.

Thyroid function on biochemistryRAIU is close to uninterpretable without this. The same 3% means destructive thyroiditis in a thyrotoxic patient and very little in a euthyroid one.

Iodine exposure in the preceding 1–2 monthsIodinated contrast suppresses uptake for one to two months, amiodarone for three to six. This is the commonest reason a RAIU comes back low in a patient who does not have thyroiditis.

Thyrotoxicosis with elevated uptakeExample

24-hour uptake 42%, 4–6 hour uptake 28%, thyrotoxic biochemistry, no iodine exposure

The differential this test exists to settle

The second row is the whole clinical value of RAIU and the reason a converter page would have been useless here. A thyrotoxic patient with a low uptake is not a mild case of the same disease as a thyrotoxic patient with a high uptake — it is a different disease with opposite management. Antithyroid drugs, radioiodine and thyroidectomy all act on a gland that is overproducing; none of them does anything for a gland that is releasing stored hormone because it is inflamed, or for a patient taking thyroxine they have not declared. Getting this wrong means months of carbimazole for a self-limiting thyroiditis.
Uptake	What it means	Causes
Elevated, or normal with a suppressed TSH	The gland is making too much hormone	Graves’ disease; toxic adenoma; toxic multinodular goitre. "Uptake is usually elevated in patients with GD and normal or high in toxic nodular goiter, unless there has been a recent exposure to iodine (e.g., radiocontrast)"
Low or near zero, with thyrotoxic biochemistry	The gland is not making the hormone — it is leaking it, or the hormone came from elsewhere	"painless, postpartum, or subacute thyroiditis; factitious ingestion of thyroid hormone; or recent excess iodine intake"; and "The uptake over the neck will also be absent in a patient with struma ovarii, where the abnormal thyroid tissue is located in an ovarian teratoma"
Low, without thyrotoxic biochemistry	Much less informative	Recent iodine load, hypothyroidism, previous thyroidectomy, external radiation, antithyroid drugs
Raised, without thyrotoxic biochemistry	Usually a trapping gland under drive	Iodine deficiency, dyshormonogenesis, early Hashimoto’s, recovery from thyroiditis, lithium

Published uptake figures, with their dates and countries attached

These are not four alternative reference intervals to pick from, and the page is not offering a "US range" and a "European range" because neither exists as a published figure. They are four measurements of the same thing in different places and decades, and the pattern in them is the point: uptake falls as dietary iodine rises. The Mayo authors state the mechanism directly — "A substantial increase in the dietary iodide intake in many communities has caused a major increase in the total iodide pool in the body and a consequent decrease in the normal thyroid radioiodine uptake" — and note that series from 1949 to 1965 had means of 20.7 to 35.9%. The UK 24-hour mean fell from 38.2% to 31.6% in eight years and then stopped falling. The current guideline draws the only safe conclusion: "Normal reference ranges vary in different regions depending on iodine intake, and need to be determined locally by each nuclear medicine laboratory", the underlying intakes being 200–700 µg/day in the USA and 80–500 µg/day in Europe.
Series	Where and when	4–6 hour	24 hour
StatPearls, sample normal range for I-123 uptake	Contemporary, United States	3–16% at 6 hours	8–25%
Mayo Clinic, 44 euthyroid patients	Rochester, Minnesota, sampled January–February 1974	9.7 ± 3.3% (range 2.5–18.9)	18.2 ± 5.3% (range 6.4–30.8)
United Kingdom series, mean uptakes	1958–59	23.5%	38.2%
Same United Kingdom series	1967	20.4%	31.6%
EANM practice guideline / SNMMI procedure standard	2019, international	No range published	"any result in excess of 25 % in iodine-sufficient regions and within the setting of clinical hyperthyroidism is compatible with thyroid hyperfunction"

How long each iodine source suppresses uptake for

From Table 3 of the EANM/SNMMI standard, with the ACR–ACNM–SNMMI–SPR practice parameter’s figures noted where they differ. A CT with contrast performed in the emergency department six weeks ago is the classic way a RAIU comes back falsely low, and the patient will not usually volunteer it — the contrast was given for something else entirely. The two guidelines disagree by a factor of ten on oil-based contrast, which is rare enough now that the discrepancy rarely bites, but is worth knowing about if a lymphangiogram or an old oil-based agent appears in the history. Note also that antithyroid drugs need only a few days’ withdrawal, which makes a diagnostic RAIU practical in a patient already started on treatment.
Source	Withdrawal interval before RAIU
Intravenous iodinated contrast	1 to 2 months (the ACR practice parameter agrees: 1–2 months)
Oil-based iodinated contrast	3 to 6 months (the ACR practice parameter says 1–2 years)
Amiodarone	3 to 6 months
Lugol’s solution or SSKI	4 to 6 weeks
Iodine-containing medicines	4 weeks
Iodine-containing antiseptics	4 weeks
Kelp	4 weeks
Levothyroxine	4 weeks
Liothyronine	2 weeks
Antithyroid drugs — methimazole, carbimazole, propylthiouracil	3 to 7 days
Perchlorate	1 week

Why this page is not a unit converter

Radioactive iodine uptake is reported as the percentage of an administered dose of radioiodine that the thyroid has trapped by a given time. A percentage has no alternative units, so there is nothing here to convert and a converter would have been a page with an empty dropdown. What people actually need when they look up a RAIU is the interpretation, and specifically one comparison: whether the uptake is high or low, in a patient whose biochemistry is already known.

The timing convention is fixed and worth stating. The EANM practice guideline and SNMMI procedure standard give the usual measurement times as 4 to 6 hours and 24 hours after the radiopharmaceutical is given, occasionally with a five-day measurement as well. The 24-hour figure is the standard; the early measurement exists chiefly to catch a specific pattern. In a small subset of hyperthyroid patients, generally with Graves’ disease, iodine turnover is fast enough that peak uptake occurs at 6 to 12 hours and the 24-hour value has already fallen away from it — though as the same standard notes, almost all of these patients still have an abnormally elevated 24-hour uptake. An early value higher than the 24-hour value is the signature, and it matters for therapy planning because rapid turnover shortens the residence time of a therapeutic dose.

Now the interpretation. In a patient with thyrotoxic biochemistry — suppressed TSH, raised free T4 or free T3 — the uptake answers whether the gland is manufacturing the excess hormone or merely spilling hormone it made earlier. The American Thyroid Association’s 2016 guideline puts both halves plainly. Uptake is usually elevated in Graves’ disease and normal or high in toxic nodular goitre, unless there has been recent iodine exposure. And the uptake will be near zero in painless, postpartum or subacute thyroiditis, in factitious ingestion of thyroid hormone, or after recent excess iodine intake — with uptake over the neck also absent in struma ovarii, where functioning thyroid tissue sits in an ovarian teratoma. Those two lists are not degrees of the same problem. The first group has a gland that is overproducing and responds to antithyroid drugs, radioiodine or surgery. The second group has a gland that is inflamed, suppressed, or not the source of the hormone at all, and none of those treatments does anything for them. A thyrotoxic patient with a low uptake is therefore the clinically important result on this page: it is the finding that stops carbimazole being started for a self-limiting thyroiditis, and it is the finding a clinician working from biochemistry alone cannot reach.

The commonest way this goes wrong is iodine. Iodinated contrast floods the body’s iodide pool and suppresses uptake for one to two months — the ACR practice parameter and the EANM/SNMMI standard agree on that interval — and amiodarone does it for three to six months. A patient who had a contrast CT in the emergency department six weeks ago will not think to mention it, because the scan was for something else. The result is a low uptake in a patient with Graves’ disease, which reads as thyroiditis and sends the management in exactly the wrong direction. If iodine exposure cannot be excluded, the honest options are to wait out the withdrawal interval or to answer the question another way: TSH receptor antibodies are unaffected by iodine and, in the ATA’s own framing, positive antibodies confirm the commonest cause of thyrotoxicosis without any radiation at all. Use the TRAb converter.

Two things this page will not give you. The first is a normal range, because there is no universal one and the guideline that would be expected to publish it explicitly declines: reference ranges vary between regions with dietary iodine intake and have to be determined locally. The intakes behind that are 200 to 700 µg per day in the USA against 80 to 500 µg per day in Europe, and the historical series in the table above show what the difference does — American euthyroid 24-hour uptakes of 6 to 31% in 1974 against United Kingdom means of 38% in 1959 and 32% in 1967, falling as iodine intake rose. The single guideline figure worth carrying is that an uptake above 25% in an iodine-sufficient region, in a patient who is clinically hyperthyroid, is compatible with hyperfunction. Everything else belongs to the laboratory that made the measurement.

The second thing is a RAIU in a pregnant patient. Radioiodine crosses the placenta and concentrates in breast milk; the ATA guideline makes the test contraindicated during pregnancy and usually during lactation, and the ACR practice parameter states that administering iodine-131 sodium iodide to pregnant or lactating patients is contraindicated. Pregnancy should be excluded before the dose, not after. In pregnancy the differential between gestational thyrotoxicosis and Graves’ disease is made on the biochemistry, on the clinical picture and on antibodies — the TSH converter, the free T4 converter and the TRAb converter — and in thyroid storm the assessment is clinical, on the Burch-Wartofsky score calculator.

Frequently asked questions

What is a normal radioactive iodine uptake?

There is no universal figure. The EANM practice guideline and SNMMI procedure standard state that normal reference ranges "vary in different regions depending on iodine intake, and need to be determined locally by each nuclear medicine laboratory". Published series give 3–16% at 6 hours and 8–25% at 24 hours in a contemporary US setting, against United Kingdom means of 20.4% at 4 hours and 31.6% at 24 hours in 1967 — the difference being dietary iodine, not method. Use the range from the laboratory that measured it.

What does a low radioactive iodine uptake in a thyrotoxic patient mean?

That the thyroid is not manufacturing the excess hormone. The 2016 ATA guideline lists the causes: "painless, postpartum, or subacute thyroiditis; factitious ingestion of thyroid hormone; or recent excess iodine intake", plus struma ovarii where functioning thyroid tissue sits in an ovarian teratoma. This is the single most useful result the test produces, because none of those conditions responds to antithyroid drugs, radioiodine or surgery — the treatments that work for Graves’ disease and toxic nodular goitre.

Which conditions cause a high radioactive iodine uptake?

Productive thyrotoxicosis. The ATA guideline states that uptake "is usually elevated in patients with GD and normal or high in toxic nodular goiter, unless there has been a recent exposure to iodine". So Graves’ disease, a toxic adenoma and a toxic multinodular goitre are the three, and the scan image separates them: diffuse uptake in Graves’, a single hot focus with the rest of the gland suppressed in a toxic adenoma, and patchy uptake in a multinodular gland.

How long after a CT with contrast can a thyroid uptake scan be done?

One to two months. Both the EANM/SNMMI standard and the ACR practice parameter give that interval for intravenous iodinated contrast; oil-based contrast agents need three to six months by one and one to two years by the other, and amiodarone needs three to six months. A contrast study the patient has forgotten about is the commonest cause of a falsely low uptake, and it produces exactly the pattern that would otherwise be read as thyroiditis.

Is a radioactive iodine uptake scan safe in pregnancy?

No — it is contraindicated. Radioiodine crosses the placenta and is concentrated in breast milk. The 2016 ATA guideline makes RAIU contraindicated during pregnancy and usually during lactation, and the ACR practice parameter states that administering iodine-131 sodium iodide to pregnant or lactating patients, whether currently breastfeeding or not, is contraindicated. Pregnancy should be excluded by serum or urine testing before any dose is given.

Why is the uptake measured at both 4–6 hours and 24 hours?

The 24-hour value is the standard measurement; the early one catches rapid iodine turnover. The EANM/SNMMI standard notes that in a small subset of hyperthyroid patients, generally with Graves’ disease, "iodine turnover is rapid that their highest thyroidal uptake is at 6–12 h; however, almost all of these patients maintain an abnormally elevated 24 h and 5 days RAIU". An early value higher than the 24-hour value identifies them, which matters when a therapeutic radioiodine dose is being planned.

Do I need a RAIU if I have already had TSH receptor antibodies measured?

Often not. The 2016 ATA guideline treats measurement of TRAb and determination of the RAIU as alternative ways of establishing the cause of thyrotoxicosis. Antibodies involve no radiation, are unaffected by a recent iodine load and can be done in pregnancy, so where the question is simply whether this is Graves’ disease they frequently answer it. RAIU still has the advantage when the question is whether a nodule is autonomous, or when a therapeutic dose is being planned.

Related calculators

References

Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016;26(10):1343–1421. "Uptake is usually elevated in patients with GD and normal or high in toxic nodular goiter, unless there has been a recent exposure to iodine (e.g., radiocontrast)"; "The RAIU will be near zero in patients with painless, postpartum, or subacute thyroiditis; factitious ingestion of thyroid hormone; or recent excess iodine intake"; "The RAIU may be low after exposure to iodinated contrast in the preceding 1–2 months"; contraindicated "during pregnancy and usually during lactation".
Giovanella L, Avram AM, Iakovou I, et al. EANM practice guideline/SNMMI procedure standard for RAIU and thyroid scintigraphy. Eur J Nucl Med Mol Imaging. 2019;46:2514–2525. "The usual time of measurement for diagnostic purposes is 4–6 h and 24 h, and occasionally at 5 days"; "Normal reference ranges vary in different regions depending on iodine intake, and need to be determined locally by each nuclear medicine laboratory"; "any result in excess of 25 % in iodine-sufficient regions and within the setting of clinical hyperthyroidism is compatible with thyroid hyperfunction"; dietary iodine intake "200 to 700 μg/day and 80–500 μg/day in the USA and Europe, respectively"; Table 3 withdrawal times.
ACR–ACNM–SNMMI–SPR practice parameter for the performance of thyroid scintigraphy and uptake measurements. Intravenous iodinated contrast 1–2 months, oil-based 1–2 years, iodine-containing medicines 4 weeks, Lugol’s or SSKI 4–6 weeks; "Administration of iodine-131 sodium iodide to pregnant or lactating patients (whether currently breastfeeding or not) is contraindicated".
StatPearls. I-123 Uptake, NCBI Bookshelf NBK559314, and Thyroid Uptake and Scan, NBK555978. Sample normal ranges "3% to 16% at 6 hours and 8% to 25% at 24 hours"; "Iodinated contrast material reduces radiotracer uptake by decreasing the number of NIS, independent of its free iodine content".
Mayo Clinic Proceedings. Thyroid radioiodine uptakes and scans in euthyroid patients. 44 euthyroid patients, Rochester, Minnesota, January–February 1974: 6-hour uptake 9.7 ± 3.3% (2.5–18.9), 24-hour 18.2 ± 5.3% (6.4–30.8); "A substantial increase in the dietary iodide intake in many communities has caused a major increase in the total iodide pool in the body and a consequent decrease in the normal thyroid radioiodine uptake".
Changes in the normal range of thyroidal radioiodine uptake, United Kingdom. Mean uptakes 23.5% at 4 hours and 38.2% at 24 hours in 1958–59, falling to 20.4% and 31.6% by 1967, with no further reduction by 1972.

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