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Quick Reference
  • Normal Range: 80-200 ng/dL (1.2-3.1 nmol/L in SI units)
  • Alternative Name: Total Triiodothyronine, T3, TT3
  • Composition: ~99.7% protein-bound, ~0.3% free (metabolically active)
  • Primary Use: Rarely ordered; specific scenarios include T3 toxicosis, amiodarone therapy, monitoring some hyperthyroid patients
  • Preferred Test: Free T3 is more accurate; not affected by binding protein changes
  • Key Point: NOT useful for hypothyroidism diagnosis; drops dramatically in critical illness (sick euthyroid syndrome)

Test Description

What is Total T3?

Total T3 measures ALL triiodothyronine in blood - both protein-bound (~99.7%) and free (~0.3%) forms. Like Total T4, Total T3 is affected by changes in binding proteins, making it less reliable than Free T3 for assessing thyroid function.

Total T3 vs. Free T3

Understanding the distinction is important for proper test selection:

  • Total T3 = Bound T3 (99.7%) + Free T3 (0.3%)
  • Free T3: Only the ~0.3% unbound fraction that is metabolically active
  • Binding proteins: Similar to T4 - mainly TBG, also transthyretin and albumin
  • TBG effects: Changes in TBG alter Total T3 but not Free T3
  • Clinical use: Free T3 is preferred; Total T3 rarely adds useful information
Total T3 is Rarely Ordered: In modern practice, Total T3 has been largely replaced by Free T3. Free T3 is more accurate because it's not affected by binding protein changes. TSH + Free T4 are sufficient for most cases; add Free T3 (not Total T3) when needed for suspected T3 toxicosis or monitoring Graves' disease.

T3 Production and Peripheral Conversion

Most circulating T3 comes from peripheral T4 conversion, not direct thyroid secretion:

  • 80% from T4 conversion: Liver, kidney, and muscle convert T4 to T3 via deiodinase enzymes
  • 20% thyroid secretion: Thyroid gland directly produces and secretes T3
  • Total T3 reflects both sources: Direct secretion plus peripheral conversion
  • Critical illness effect: Dramatically decreased T4-to-T3 conversion causes low Total T3 (sick euthyroid syndrome)

When Total T3 Might Be Ordered

Total T3 has very limited indications in modern practice:

  • T3 toxicosis: Suspected hyperthyroidism with normal Free T4 (Free T3 is actually preferred)
  • Amiodarone monitoring: Assessing T4-to-T3 conversion blockade (Free T3 also acceptable)
  • Resource-limited settings: If Free T3 assay unavailable (rare in developed countries)
  • Historical comparison: Following trends in patients with prior Total T3 measurements
Why Free T3 is Better: Free T3 measures only the metabolically active hormone and is not affected by TBG variations. This makes Free T3 superior to Total T3 for the same reasons Free T4 is superior to Total T4. If you're considering ordering Total T3, order Free T3 instead.

Total T3 is NOT Useful for Hypothyroidism

Total T3 should NEVER be used to diagnose hypothyroidism:

  • Compensatory mechanism: In hypothyroidism, increased T4-to-T3 conversion maintains normal T3 levels
  • Late decline: Total T3 only drops in severe, advanced hypothyroidism
  • Misleading reassurance: Normal Total T3 does NOT exclude hypothyroidism
  • Proper tests: Use TSH + Free T4 for hypothyroidism diagnosis, never Total T3
Normal Ranges

Total T3 reference ranges vary by age and clinical condition. Critical illness dramatically lowers Total T3.

Swipe to see more
Population Total T3 (ng/dL) SI Units (nmol/L)
Adults 80-200 1.2-3.1
Elderly (>65 years) 70-180 1.1-2.8
Pregnancy 100-260 1.5-4.0
Children (1-15 years) 105-245 1.6-3.8
Newborns (1-3 days) 100-740 1.5-11.4
Important Considerations:
  • Conversion factor: ng/dL × 0.0154 = nmol/L
  • Assay variation: Total T3 reference ranges vary between laboratories and assay methods
  • Pregnancy increase: Total T3 rises with TBG increase during pregnancy (like Total T4)
  • Age decline: Total T3 decreases with age; lower levels normal in elderly
  • Critical illness: Total T3 drops dramatically in sick patients; low T3 does NOT indicate hypothyroidism
Clinical Significance

Elevated Total T3

True Hyperthyroidism (High Total T3, High Free T3, Low TSH)

  • T3 toxicosis: Hyperthyroidism with disproportionately elevated T3 vs. T4
  • Early Graves' disease: T3 may rise before T4 in early autoimmune hyperthyroidism
  • Toxic nodular goiter: Autonomous nodules may preferentially secrete T3
  • Toxic adenoma: Single autonomous nodule producing excess T3
  • Graves' disease (general): Both T4 and T3 elevated
  • Thyroiditis: Transient thyrotoxicosis from stored hormone release

T3 Elevation Can Occur Before T4 in Early Hyperthyroidism

  • Early detection: Some hyperthyroid patients have elevated Total T3/Free T3 while Free T4 still normal
  • Pattern: Low TSH, normal Free T4, high Free T3/Total T3
  • Clinical relevance: Patient is thyrotoxic despite normal T4; diagnosis missed without T3 measurement
  • Preferred test: Free T3 better than Total T3 for this indication

Low Total T3

Non-Thyroidal Illness (Sick Euthyroid Syndrome) - MOST COMMON CAUSE

Critical Illness Causes Dramatic T3 Drop:
  • Pattern: Low Total T3 (often 20-60 ng/dL), low/normal T4, low/normal TSH
  • Mechanism: Decreased Type 1 deiodinase activity; impaired T4-to-T3 conversion
  • Adaptive response: Energy conservation during severe illness; protective, not pathologic
  • Severity correlation: Lower T3 correlates with worse prognosis in critical illness
  • CRITICAL - DO NOT TREAT: Thyroid hormone replacement in critically ill patients increases mortality
  • Management: Recheck thyroid function 6-8 weeks after illness resolves

Hypothyroidism (Usually Normal Total T3 Until Advanced)

  • Mild-moderate hypothyroidism: Total T3 often NORMAL due to compensatory T4-to-T3 conversion
  • Severe hypothyroidism: Total T3 finally drops in advanced disease
  • Key point: Normal Total T3 does NOT exclude hypothyroidism
  • Proper diagnosis: Use TSH + Free T4, NEVER Total T3, for hypothyroid diagnosis

Medications Blocking T4-to-T3 Conversion

  • Amiodarone: Blocks Type 1 deiodinase; high Total T4, low Total T3
  • Propranolol (high dose): >160 mg/day inhibits conversion
  • Propylthiouracil (PTU): Blocks conversion in addition to synthesis inhibition
  • Glucocorticoids (high dose): Dexamethasone, prednisone >60 mg/day suppress deiodinase
  • Iodinated contrast agents: Iopanoic acid, ipodate (rarely used now)

Physiological States Lowering Total T3

  • Fasting/starvation: Decreased T4-to-T3 conversion; adaptive energy conservation
  • Chronic caloric restriction: Prolonged dieting lowers Total T3
  • Aging: Total T3 decreases with age; normal variant in elderly
  • Selenium deficiency: Deiodinase enzymes require selenium; rare in US

Total T3 in Specific Clinical Scenarios

Amiodarone-Induced Thyroid Dysfunction

  • Expected pattern in euthyroid patients: High Total T4/Free T4, low Total T3/Free T3, normal/slightly elevated TSH
  • Amiodarone-induced thyrotoxicosis (AIT): Type 1 (iodine-induced) vs. Type 2 (destructive); Total T3 helps distinguish
  • Type 1 AIT: High Total T4 AND high Total T3; treat with antithyroid drugs
  • Type 2 AIT: High Total T4, normal/low Total T3; treat with corticosteroids
  • Note: Free T3 can also be used instead of Total T3
Interpretation Guidelines

Total T3 Interpretation Patterns

Swipe to see more
Total T3 Total T4 TSH Interpretation
High High Low Hyperthyroidism (Graves', toxic nodular goiter)
High Normal Low T3 Toxicosis (early Graves', toxic nodule)
Low Low/Normal Low/Normal Sick Euthyroid Syndrome (critical illness)
Low High Normal/High Amiodarone effect or impaired T4-to-T3 conversion
Normal Low High Hypothyroidism (compensated T3; do NOT rule out hypothyroidism)
Normal Normal Normal Euthyroid (Normal)
Common Misinterpretation - Normal Total T3 in Hypothyroidism: Many clinicians incorrectly assume normal Total T3 excludes hypothyroidism. This is WRONG. In mild-moderate hypothyroidism, compensatory T4-to-T3 conversion keeps T3 normal. Total T3 only drops in severe hypothyroidism. NEVER use Total T3 to rule out hypothyroidism - use TSH + Free T4.

Total T3 in Critical Illness (Sick Euthyroid Syndrome)

Sick Euthyroid Syndrome Pattern:
  • Mild illness: Total T3 low (60-80 ng/dL), Total T4 normal, TSH normal
  • Moderate illness: Total T3 very low (30-60 ng/dL), Total T4 low/normal, TSH low/normal
  • Severe illness: Total T3 very low (<30 ng/dL), Total T4 low, TSH low; reverse T3 (rT3) elevated
  • Recovery phase: TSH may transiently rise (can mimic hypothyroidism)
  • Management: DO NOT TREAT with thyroid hormone; increases mortality. Recheck 6-8 weeks post-recovery

When to Use Total T3 vs. Free T3

Test Selection Guidance:
  • Routine assessment: Use TSH + Free T4; rarely need any T3 test
  • Suspected T3 toxicosis: Free T3 preferred over Total T3
  • Monitoring Graves' treatment: Free T3 preferred over Total T3
  • Amiodarone monitoring: Either Free T3 or Total T3 acceptable
  • Pregnancy: Use Free T3 (Total T3 elevated due to increased TBG)
  • Bottom line: If you need to measure T3, order Free T3, not Total T3, in nearly all situations
Interfering Factors

Factors That Increase Total T3

True Hyperthyroidism:

  • Graves' disease, toxic multinodular goiter, toxic adenoma, T3 toxicosis
  • Thyroiditis (subacute, postpartum, silent) - transient elevation

Increased TBG (Total T3 High, Free T3 Normal):

  • Pregnancy: Estrogen increases TBG; Total T3 rises
  • Oral contraceptives/estrogen therapy: Similar mechanism
  • Acute hepatitis: Transiently increased TBG synthesis
  • Inherited TBG excess: X-linked genetic variant

Other Causes:

  • High altitude: Increased T4-to-T3 conversion
  • Heparin therapy: May falsely elevate Total T3 in some assays (in vitro artifact)

Factors That Decrease Total T3

Non-Thyroidal Illness (Most Common Cause of Low Total T3):

  • Sepsis, pneumonia, myocardial infarction, stroke, trauma, surgery, burns
  • Chronic kidney disease, liver failure, heart failure
  • Diabetic ketoacidosis, severe infections
  • Cancer, malnutrition, anorexia nervosa
  • Mechanism: Decreased Type 1 deiodinase; reduced T4-to-T3 conversion

Medications Blocking T4-to-T3 Conversion:

  • Amiodarone: Potent blocker of Type 1 deiodinase
  • Propranolol: High doses (>160 mg/day) inhibit conversion
  • Propylthiouracil (PTU): Blocks synthesis AND conversion
  • Glucocorticoids: High doses (dexamethasone, prednisone >60 mg/day)
  • Iodinated contrast: Iopanoic acid, ipodate (rarely used)

Decreased TBG (Total T3 Low, Free T3 Normal):

  • Nephrotic syndrome: Urinary TBG loss
  • Cirrhosis: Decreased hepatic TBG synthesis
  • Androgens/anabolic steroids: Suppress TBG production
  • Glucocorticoids: Decrease TBG synthesis
  • Inherited TBG deficiency: X-linked disorder

Physiological States:

  • Fasting/starvation: Decreased T4-to-T3 conversion; Total T3 can drop 50%
  • Chronic caloric restriction: Prolonged dieting lowers Total T3
  • Aging: Total T3 decreases with age (normal variant)
  • Selenium deficiency: Deiodinase enzymes require selenium

Assay Interference

  • Biotin (high dose): >5 mg/day may interfere with immunoassays; stop 48-72 hours before testing
  • Heterophile antibodies: Can cause falsely high or low results
  • Anti-T3 antibodies: Rare autoantibodies interfere with Total T3 measurement
Clinical Pearls
"Total T3 is rarely ordered - use Free T3 instead": Total T3 suffers from the same binding protein issues as Total T4. Free T3 is more accurate and not affected by TBG changes. If you're considering Total T3, order Free T3 instead. Better yet, TSH + Free T4 are sufficient for most cases.
Clinical Pearl
"Total T3 is useless for hypothyroidism diagnosis": In hypothyroidism, compensatory T4-to-T3 conversion maintains normal Total T3 until disease is severe. Normal Total T3 does NOT exclude hypothyroidism. ALWAYS use TSH + Free T4, NEVER Total T3, for hypothyroid diagnosis.
"Low Total T3 in ICU = sick euthyroid syndrome - DO NOT TREAT": Critical illness causes Total T3 to drop dramatically (often <40 ng/dL) without true hypothyroidism. This is adaptive energy conservation. Treating with thyroid hormone INCREASES mortality. Recheck 6-8 weeks after recovery.
Clinical Pearl
"Total T3 can be elevated in early hyperthyroidism before T4 rises": Some hyperthyroid patients (especially early Graves' or toxic nodules) have elevated Total T3 while Total T4 is still normal. This is T3 toxicosis. However, Free T3 is preferred over Total T3 for this indication.
Amiodarone pattern with Total T3: Amiodarone blocks T4-to-T3 conversion, causing high Total T4, low Total T3, and normal/slightly elevated TSH in euthyroid patients. This is expected, not thyroid disease. However, amiodarone can ALSO cause true hypo- or hyperthyroidism.
Clinical Pearl
"Total T3 drops 50% with fasting": Caloric restriction and fasting decrease T4-to-T3 conversion as adaptive energy conservation. Total T3 can drop from 120 to 60 ng/dL with prolonged fasting. This causes fatigue and metabolic slowing during dieting. Not pathologic - resolves with refeeding.
Clinical Pearl
Pregnancy increases Total T3 (like Total T4): Estrogen-induced TBG increase raises Total T3 during pregnancy. Free T3 remains normal. Don't misinterpret elevated Total T3 in pregnant women as hyperthyroidism - check TSH and Free T4/Free T3.
Total T3 useful for distinguishing amiodarone-induced thyrotoxicosis types: Type 1 AIT (iodine-induced) has high Total T4 AND high Total T3. Type 2 AIT (destructive thyroiditis) has high Total T4 but normal/low Total T3. This distinction guides treatment (antithyroid drugs vs. corticosteroids).
Clinical Pearl
"Lower Total T3 = worse prognosis in critical illness": In ICU patients, severity of Total T3 suppression correlates with mortality. Total T3 <30 ng/dL indicates severe illness. However, this is prognostic, not a treatment target. Do NOT give thyroid hormone to raise T3.
Clinical Pearl
PTU blocks T4-to-T3 conversion; methimazole doesn't: Propylthiouracil inhibits both thyroid hormone synthesis AND peripheral T4-to-T3 conversion, lowering Total T3 faster. Methimazole only blocks synthesis. This is why PTU may be preferred in thyroid storm (rapid effect on circulating T3).
Never order Total T3 to "complete" a thyroid panel: Total T3 rarely adds useful information beyond TSH, Free T4, and Free T3 (when indicated). It's not part of routine thyroid screening. Order it only for specific indications (T3 toxicosis, amiodarone monitoring) - and even then, Free T3 is usually preferred.
Clinical Pearl
Total T3 drops with age: Elderly patients have lower Total T3 (70-180 ng/dL) due to decreased T4-to-T3 conversion and reduced metabolic demands. This is physiologic aging, not hypothyroidism. TSH and Free T4 are better for assessing thyroid function in elderly.
Selenium deficiency lowers Total T3: Deiodinase enzymes (which convert T4 to T3) are selenoproteins requiring selenium. Severe selenium deficiency impairs T4-to-T3 conversion, lowering Total T3. Rare in US but seen in certain malnutrition states or specific geographic regions.
References
  1. Kratz, A., Ferraro, M., Sluss, P. M., & Lewandrowski, K. B. (2004). Laboratory reference values. New England Journal of Medicine, 351, 1548-1564.
  2. Lee, M. (Ed.). (2009). Basic skills in interpreting laboratory data. Ashp.
  3. Farinde, A. (2021). Lab values, normal adult: Laboratory reference ranges in healthy adults. Medscape. https://emedicine.medscape.com/article/2172316-overview?form=fpf
  4. Nickson, C. (n.d.). Critical Care Compendium. Life in the Fast Lane • LITFL. https://litfl.com/ccc-critical-care-compendium/
  5. Farkas, Josh MD. (2015). Table of Contents - EMCrit Project. EMCrit Project. https://emcrit.org/ibcc/toc/
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