Bedside Snapshot
- Normal (Adult)
- Typical 78–158 ng/dL (1.2–2.4 nmol/L) per MedlinePlus, assay-specific. Textbook tables give 60–180 (Critical Care Transport) and 59–156 ng/dL (Katzung). The lab's own range and flag override
- Critical Values
- No universal critical value: interpret by trend and clinical context
- High Suggests
- T3 toxicosis (T3 high, T4 normal, TSH low)
- Graves' disease or toxic nodule
- Pregnancy or estrogen (raised TBG; free T3 usually normal)
- Low Suggests
- Rarely diagnostic alone.
- Nonthyroidal illness (sick euthyroid)
- Fasting or caloric restriction
- Amiodarone, PTU or high-dose glucocorticoids
- Severe hypothyroidism (late)
- Act Now If
- Suspected thyroid storm: treat per protocol and notify medical control or the receiving team; do not wait for a T3 result
- Watch Out For
- Ordering total T3 to rule out hypothyroidism: a normal result does not exclude it; use TSH and free T4
Reference & Critical Ranges
Lab-specific ranges: Published adult ranges differ between sources. The numbers below come from MedlinePlus. Your lab's reference range and flag win.
Reference ranges
| Population | Conventional | SI | Notes |
|---|---|---|---|
| Adult | 78–158 ng/dL | 1.2–2.4 nmol/L | MedlinePlus adult range. Critical Care Transport Table 18-3 gives 60–180 ng/dL and Katzung 15e Table 38-2 59–156 ng/dL (0.9–2.4 nmol/L), so labs differ: use the lab's own range |
| Pregnancy | Higher than non-pregnant | — | Use the lab's pregnancy range; estrogen raises TBG |
| Older adult | Use the lab's age range | — | No verified universal cut-off |
| Children and adolescents | Age-specific | — | Use the lab's pediatric range (age-specific under 20) |
Severity & critical values
| Level | Conventional | SI | What it means |
|---|---|---|---|
| Low | <78 ng/dL | <1.2 nmol/L | Illness, fasting, drug effect on conversion, or severe hypothyroidism. Low alone does not diagnose hypothyroidism |
| Normal | 78–158 ng/dL | 1.2–2.4 nmol/L | MedlinePlus adult range; read with TSH and free T4 |
| High | >158 ng/dL | >2.4 nmol/L | T3 excess (T3 toxicosis, Graves', toxic nodule) or raised TBG |
| Critical | No universal value | No universal value | Interpret by trend, TSH, free T4 and clinical context |
Critical values: No universal critical value exists for total T3. A low or high result supports the picture. It is not a panic value. Act on the thyroid status that TSH, free T4 and the patient show.
Conversion: ng/dL × 0.0154 = nmol/L
Causes of Abnormal Results
High (above 158 ng/dL)
| Mechanism | Common causes | Bedside clue |
|---|---|---|
| Excess thyroid hormone secretion | Graves' disease, toxic multinodular goiter, toxic adenoma | Low TSH; T3 disproportionately high relative to T4 in T3 toxicosis |
| Destructive thyroiditis | Subacute, postpartum or silent thyroiditis | Transient thyrotoxicosis from stored hormone release |
| Amiodarone-induced thyrotoxicosis | Type 1 (iodine-induced) or type 2 (destructive) | High T4 with thyroid symptoms; T3 does not separate the types (see Interpretation) |
| Exogenous T3 (liothyronine) | Liothyronine (Cytomel) or other T3 products; larger doses can be toxic, especially with sympathomimetic amines | Ask about prescriptions and supplements; TSH suppressed |
| Raised TBG | Pregnancy, estrogen or oral contraceptives, inherited TBG excess | High total T3 with normal free T3 and TSH |
| Assay interference | High-dose biotin, heterophile or anti-T3 antibodies | Result does not fit the clinical picture; check supplements |
Low (below 78 ng/dL)
Low total T3 is common in illness. Alone, it does not mean hypothyroidism.
| Mechanism | Common causes | Bedside clue |
|---|---|---|
| Nonthyroidal illness (sick euthyroid) | Sepsis, trauma, surgery, myocardial infarction, stroke, liver or kidney failure, severe illness | Low T3 with normal or low T4 and normal or low TSH |
| Blocked T4-to-T3 conversion | Amiodarone, propylthiouracil, high-dose glucocorticoids, high-dose propranolol | Low T3 relative to T4 (amiodarone raises T4; PTU is given for hyperthyroidism, so T4 may be normal or low) |
| Fasting or caloric restriction | Starvation, prolonged dieting | Low T3 with normal TSH; recovers with refeeding |
| Severe or advanced hypothyroidism | Late primary hypothyroidism | Low T3 with high TSH and low T4 |
| Low TBG | Nephrotic syndrome (urinary protein loss), androgens, glucocorticoids, severe hepatic synthetic failure | Low total T3 with normal free T3 |
The liver makes TBG. Acute hepatitis can raise it, so liver disease does not always pull total T3 down.
Interpretation & Bedside Approach
Step-by-step approach
- Is the test needed? TSH and free T4 answer most thyroid questions. Order total T3 only for a specific question, such as suspected T3 toxicosis.
- Check for interference: Ask about high-dose biotin and other supplements before accepting a result that does not fit the patient.
- Read T3 with TSH and free T4: Use the pattern table below.
- Consider binding protein: Pregnancy, estrogen and other TBG changes move total T3 without changing free hormone.
- Consider illness and drugs: Acute illness, fasting, amiodarone, propylthiouracil and high-dose glucocorticoids lower T4-to-T3 conversion.
- Escalate: For suspected thyroid storm or an unexplained pattern in a sick patient, involve endocrinology. Do not start thyroid hormone because of a low T3 alone.
Total T3 pattern table
| Total T3 | Free T4 | TSH | Suggests |
|---|---|---|---|
| High | High | Low | Hyperthyroidism (Graves', toxic nodules, thyroiditis) |
| High | Normal | Low | T3 toxicosis |
| Low | Normal or low | Normal or low | Nonthyroidal illness, fasting or a drug effect |
| Low | High | Normal or high | Amiodarone or another conversion blocker |
| Low | Low | High | Severe hypothyroidism |
| Normal | Low | High | Hypothyroidism; normal T3 does not exclude it |
| Normal | Normal | Low | Subclinical hyperthyroidism, or early or T3-predominant disease; correlate clinically and repeat |
| Normal | Normal | Normal | Euthyroid |
Liothyronine monitoring
- Secondary or tertiary hypothyroidism: TSH is not a reliable measure of dose adequacy. Don't use it to monitor therapy. The label says to monitor with serum T3 and keep it in the normal range.
- Adult primary hypothyroidism: Monitor TSH. Per the label, total T3 is most appropriate to check an early response, before TSH has had time to change.
- Draw timing: The label gives no draw time. Ask the prescriber.
Amiodarone types: The 2018 European Thyroid Association guideline says absolute free T4 and free T3 at presentation do not tell type 1 from type 2 amiodarone-induced thyrotoxicosis. That question needs the specialist work-up: thyroid antibodies, color-flow Doppler, radioiodine uptake.
Special Populations
| Population | What changes | Clinical implication |
|---|---|---|
| Neonates and infants | Values differ from adult values and change over the first weeks of life | Use the lab's neonatal range, not the adult range |
| Children and adolescents | Values are age-specific under 20 | Compare with the lab's pediatric range for age |
| Pregnancy | Estrogen raises TBG, so total T3 rises while free T3 usually stays near normal | Use the pregnancy range; a high total T3 alone is not hyperthyroidism |
| Older adults | Values can be lower; no verified universal cut-off | Interpret with the lab's age range; low T3 during acute illness is usually illness-related |
| CKD, dialysis and liver disease | Illness and reduced binding protein lower total T3 | Read with free T4 and TSH; do not treat the number alone |
Interfering Factors & Pitfalls
Medications and supplements
| Factor | Effect | Why | Avoid or fix |
|---|---|---|---|
| High-dose biotin | Competitive assays (total T3, T4, free T4): falsely ↑; sandwich assays (TSH): falsely ↓ | Interferes with streptavidin-biotin immunoassays | Ask about supplements before the draw; follow lab advice on stopping |
| Amiodarone, propylthiouracil, high-dose glucocorticoids | ↓ T3 | Block T4-to-T3 conversion | Interpret with free T4 and TSH |
| Liothyronine (exogenous T3) | ↑ total and free T3 | Exogenous hormone | Ask about T3 products before interpreting |
| Estrogen or pregnancy | ↑ total T3 | Raises TBG | Use the pregnancy or estrogen-adjusted range |
| Fasting | ↓ T3 | Reduced conversion | Note the fasting state when interpreting |
Assay and laboratory differences
- Method and range: Analyzers give different ranges. Keep trends on one lab's method.
- Biotin: Competitive assays (total T3, T4, free T4) tend to read biotin as falsely high. Sandwich assays (TSH) read it as falsely low. Ask the lab which assay it runs. No firm washout time has been established.
Common misinterpretations
- Normal total T3 excludes hypothyroidism: No. Reach for TSH and free T4.
- Low T3 in an ICU patient means hypothyroidism: Most of the time it's the illness. Treating the number is not supported.
- High total T3 in pregnancy means hyperthyroidism: Usually it's raised TBG.
- T3 separates amiodarone thyrotoxicosis types: The ETA 2018 guideline says it doesn't.
Biotin: Before a thyroid draw, ask about high-dose biotin. It can falsely raise total T3 and free T4, and falsely lower TSH at the same time. That pattern can look like Graves' disease.
Physiology & Test Basics
What it measures
Total T3 counts all the triiodothyronine in serum. Most of it is protein-bound (more than 99%), with a small free fraction alongside. Free T3 is a separate test. The two are not interchangeable.
Physiology & regulation
- Source: The thyroid mostly secretes T4. Most circulating T3 is made out in the periphery, as deiodinase enzymes in liver, kidney and muscle convert T4. The thyroid adds a smaller direct share.
- Binding: Thyroxine-binding globulin (TBG) carries most of the T3. Albumin and transthyretin carry the rest.
- Free hormone is active: Only the free fraction gets into cells. Total T3 rides on TBG, so it can change while free T3 stays normal.
- Feedback: TSH falls when T3 and T4 rise. It rises when they fall.
- Suppressors: Acute illness, fasting and deiodinase-blocking drugs turn conversion down.
How it is measured
Most labs run it on serum by immunoassay, from a red-top or serum separator tube, per lab. Results depend on the method, so compare against the same lab. Fasting itself lowers T3. Note the fasting state.
Why it matters
When TSH and free T4 are available, total T3 rarely changes management. Order it for suspected T3 toxicosis, when free T4 is normal and TSH is suppressed. Order it, too, to keep a trend going when a prior total T3 was measured. It also helps you recognize amiodarone-related thyroid disease. And a low T3 in critical illness is not hypothyroidism.
Clinical Pearls
Low T3 in critical illness is not hypothyroidism: Nobody has settled whether thyroid hormone helps in nonthyroidal illness. Don't start it because of a low total T3 alone. Decide with endocrinology, then reassess thyroid function once the illness resolves.
Amiodarone changes the pattern, not the diagnosis: Amiodarone blocks T4-to-T3 conversion, so expect high free T4 with low total T3. The thyroid problem still needs its own work-up.
Amiodarone type decides the treatment: Type 1 gets antithyroid drugs. The ETA 2018 guideline names methimazole, carbimazole and propylthiouracil. Type 2 gets oral glucocorticoids first, for moderate-to-severe thyrotoxicosis.
Pregnancy raises total T3 through TBG: Before you call it hyperthyroidism, check free T4 and TSH against the pregnancy range.
PTU also blocks T4-to-T3 conversion: Propylthiouracil blocks peripheral conversion as well as synthesis. Methimazole acts on synthesis. The ATA 2016 thyroid storm table lists PTU.
Liothyronine can mimic hyperthyroidism: Before you read a high T3 or a suppressed TSH, ask about liothyronine (Cytomel) and other T3 products. Larger doses can cause serious or life-threatening toxicity, especially with sympathomimetic amines, the label warns. Put the product on the handoff.
Field relevance: You won't use total T3 in the field. Suspected thyroid storm (fever, tachycardia, agitation)? Treat per protocol and transport. Don't hold care waiting on a T3 result.
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. https://pubmed.ncbi.nlm.nih.gov/27521067/
- Bartalena L, Bogazzi F, Chiovato L, et al. 2018 guidelines for the management of amiodarone-associated thyroid dysfunction. European Thyroid Journal. 2018;7(2):55–66. https://pmc.ncbi.nlm.nih.gov/articles/PMC5869486
- Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force. Thyroid. 2014;24(12):1670–1751. https://pubmed.ncbi.nlm.nih.gov/25266247/
- Alexander EK, Pearce EN, Brent GA, et al. 2017 guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017;27(3):315–389. https://pubmed.ncbi.nlm.nih.gov/28056690/
- Sciacchitano S, Capalbo C, Napoli C, et al. Nonthyroidal illness syndrome: to treat or not to treat? Have we answered the question? A review of metanalyses. Frontiers in Endocrinology. 2022;13:850328. https://doi.org/10.3389/fendo.2022.850328
- MedlinePlus (National Library of Medicine). Triiodothyronine (T3) test. Review date 1/25/2026. https://medlineplus.gov/ency/article/003687.htm
- Liothyronine sodium tablets, USP, prescribing information (Sigmapharm Laboratories). DailyMed, updated January 10, 2023. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=38ef3766-bb9a-402e-9e4a-ad8bb5b26e4c
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