Medical Disclaimer
  • For Educational Purposes Only: This content is intended for educational reference and should not be used for clinical decision-making.
  • Not a Substitute for Professional Judgment: Always consult your local protocols, institutional guidelines, and supervising physicians.
  • Accuracy Not Guaranteed: While all content has been prepared to the best of my knowledge and ability, errors or omissions may exist.
  • Verify Before Acting: Users are responsible for verifying information through authoritative sources before any clinical application.
AI Assistance Notice
The clinical content and references are curated and reviewed by myself; however, AI was used to assist in organizing, paraphrasing, and formatting the information presented.
Quick Reference
  • Total T3 Range: 80-200 ng/dL (1.2-3.1 nmol/L)
  • Free T3 Range: 2.3-4.2 pg/mL (3.5-6.5 pmol/L)
  • Half-life: ~1 day (much shorter than T4)
  • Primary Use: Diagnose hyperthyroidism, especially T3 toxicosis
  • Sample Type: Serum
  • Key Point: T3 is 3-4x more potent than T4 but present in much lower concentrations

Test Description

What is T3?

Triiodothyronine (T3) is the most metabolically active thyroid hormone. Although the thyroid gland primarily secretes thyroxine (T4), about 80% of circulating T3 is produced by peripheral conversion of T4 to T3 by deiodinase enzymes in target tissues.

T3 Physiology

  • Production: ~20% directly from thyroid, ~80% from peripheral T4→T3 conversion
  • Potency: 3-4 times more biologically active than T4
  • Protein binding: >99% bound to TBG, transthyretin, and albumin
  • Free T3: Only ~0.3% is unbound and biologically active
  • Half-life: ~1 day (vs 7 days for T4)

Total T3 vs Free T3

Swipe to see more
Feature Total T3 Free T3
Measures Bound + unbound T3 Only unbound (active) T3
Affected by TBG Yes - significantly No - unaffected
Clinical utility More widely available More accurate in altered TBG states
When to Order T3: T3 is not part of routine thyroid screening. Order T3 when TSH is suppressed but free T4 is normal (to detect T3 toxicosis), or when hyperthyroidism is suspected but free T4 is only marginally elevated.
Normal Ranges
Swipe to see more
Test Normal Range SI Units
Total T3 80-200 ng/dL 1.2-3.1 nmol/L
Free T3 2.3-4.2 pg/mL 3.5-6.5 pmol/L

Age and Condition Variations

  • Elderly: T3 levels decline with age (~10-15% lower in elderly)
  • Pregnancy: Total T3 increases due to elevated TBG; Free T3 remains stable
  • Fasting/illness: T3 decreases ("euthyroid sick syndrome")
  • Children: T3 levels are higher in children than adults
Critical Values: There are no universally defined critical T3 values, but severely elevated T3 (>400 ng/dL) in the setting of suppressed TSH suggests thyroid storm risk.
Clinical Significance

Elevated T3

T3 Toxicosis:

  • Hyperthyroidism with elevated T3 but normal or only mildly elevated T4
  • Occurs in 5% of hyperthyroid patients
  • Common in early Graves' disease, toxic nodular goiter, and iodine-induced hyperthyroidism
  • TSH will be suppressed

Classic Hyperthyroidism:

  • Graves' disease: Most common cause; both T3 and T4 elevated
  • Toxic multinodular goiter: T3 often elevated out of proportion to T4
  • Toxic adenoma: Single autonomous nodule
  • Subacute thyroiditis: Transient elevation during inflammatory phase

Other Causes of Elevated T3:

  • Elevated TBG: Pregnancy, estrogen therapy, hepatitis (raises Total T3 only)
  • Exogenous T3: Liothyronine (Cytomel) use/abuse
  • Familial dysalbuminemic hyperthyroxinemia: Can affect T3 assays

Low T3

Euthyroid Sick Syndrome (Non-Thyroidal Illness):

  • Most common cause of low T3 in hospitalized patients
  • Decreased peripheral T4→T3 conversion during illness
  • T3 low, T4 normal or low, TSH usually normal
  • Not true hypothyroidism - do not treat with thyroid hormone
  • Resolves when underlying illness resolves

Primary Hypothyroidism:

  • T3 is NOT recommended for diagnosing hypothyroidism
  • T3 maintained until late in hypothyroidism due to increased T4→T3 conversion
  • TSH and Free T4 are the appropriate tests

Other Causes of Low T3:

  • Fasting/caloric restriction: Adaptive decrease to conserve energy
  • Medications: Amiodarone, propranolol, glucocorticoids, propylthiouracil
  • Low TBG: Androgens, nephrotic syndrome, severe illness
  • Selenium deficiency: Required for deiodinase enzyme function
Interpretation Guidelines

When T3 is Most Useful

  • Suppressed TSH + Normal Free T4: Check T3 to diagnose T3 toxicosis
  • Hyperthyroid symptoms with equivocal labs: T3 may be elevated when T4 is borderline
  • Monitoring Graves' disease treatment: T3 can remain elevated longer than T4
  • Thyroid hormone replacement monitoring: If on T3 (liothyronine) therapy

When NOT to Use T3

  • Hypothyroidism screening/diagnosis: Use TSH and Free T4 instead
  • Hospitalized/acutely ill patients: Euthyroid sick syndrome confounds interpretation
  • Routine thyroid screening: T3 adds cost without improving sensitivity

Pattern Recognition

Swipe to see more
TSH Free T4 T3 Interpretation
↓↓ Normal ↑ T3 toxicosis
↓↓ ↑ ↑ Overt hyperthyroidism
Normal Normal/↓ ↓ Euthyroid sick syndrome
↑↑ ↓ ↓ or Normal Primary hypothyroidism
Interfering Factors

Conditions Affecting Total T3 (TBG-Dependent)

Increased TBG (↑ Total T3, normal Free T3):

  • Pregnancy
  • Estrogen/OCP use
  • Hepatitis
  • Acute intermittent porphyria
  • Genetic TBG excess

Decreased TBG (↓ Total T3, normal Free T3):

  • Androgens, anabolic steroids
  • Nephrotic syndrome
  • Chronic liver disease
  • Severe illness/malnutrition
  • Genetic TBG deficiency

Medications Affecting T3 Levels

  • Decrease T3: Amiodarone, propranolol, glucocorticoids, propylthiouracil, iodinated contrast
  • Increase T3: Amiodarone (can cause hyper or hypo), heparin (assay artifact)

Assay Interference

  • Biotin: High-dose biotin supplements can interfere with immunoassays (falsely elevated or decreased depending on assay)
  • Heterophilic antibodies: Rare cause of spurious results
Clinical Pearls
Clinical Pearl
T3 is not for hypothyroidism: Do not check T3 to diagnose hypothyroidism. The body preferentially maintains T3 levels by increasing T4→T3 conversion, so T3 may be normal until late in hypothyroidism. TSH and Free T4 are the correct tests.
Clinical Pearl
T3 toxicosis clue: If a patient has classic hyperthyroid symptoms (tachycardia, tremor, weight loss) with suppressed TSH but normal Free T4, always check T3. T3 toxicosis is more common in early Graves' disease and toxic nodular goiter.
Clinical Pearl
Sick euthyroid: In hospitalized patients, low T3 is usually euthyroid sick syndrome, not hypothyroidism. Unless TSH is markedly elevated (>10), avoid initiating thyroid hormone replacement in the acute setting.
Thyroid storm: In thyroid storm, T3 and T4 are elevated, but the degree of elevation does not correlate with severity - diagnosis is clinical. Treatment includes beta-blockers, PTU (blocks T4→T3 conversion), iodine (blocks release), and steroids.
T3 therapy monitoring: If a patient is on liothyronine (synthetic T3), T3 levels will fluctuate significantly throughout the day due to the short half-life. Check T3 at a consistent time relative to the dose.
References
  1. Garber, J. R., et al. (2012). Clinical practice guidelines for hypothyroidism in adults. Thyroid, 22(12), 1200-1235.
  2. Ross, D. S., et al. (2016). 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid, 26(10), 1343-1421.
  3. Jonklaas, J., et al. (2014). Guidelines for the treatment of hypothyroidism. Thyroid, 24(12), 1670-1751.
  4. Kaptein, E. M. (1996). Clinical application of free thyroxine determinations. Clinics in Laboratory Medicine, 16(4), 681-700.
  5. Peeters, R. P., et al. (2003). Reduced activation and increased inactivation of thyroid hormone in tissues of critically ill patients. Journal of Clinical Endocrinology & Metabolism, 88(7), 3202-3211.
Back to Thyroid Panel All Lab Values