Triiodothyronine (T3 / FT3)
Clinical interpretation, reference ranges, and thyroid status evaluation for Triiodothyronine (T3/FT3) under ATA 2017 standards.
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Quick Reference Facts
Laboratory Interpretation Workflow
1. Clinical Indication
Ordering protocols based on clinical indications, presenting symptoms, or screening guidelines.
Clinical Pearl: Order when screening for primary organ dysfunctions or monitoring active treatment efficacy.
Test Description & Overview
Triiodothyronine (T3): The active metabolic thyroid hormone produced predominantly via 5'-deiodination of Thyroxine (T4) in peripheral tissues (80%) and direct thyroid follicular secretion (20%). Free T3 (FT3) measures the metabolically active unbound fraction.
Free T3 (FT3): 2.0 - 4.4 pg/mL (3.1 - 6.8 pmol/L); Total T3: 80 - 200 ng/dL (1.2 - 3.1 nmol/L).
Elevated (High) Values
- Hyperthyroidism (Graves' Disease, Toxic Multinodular Goiter)
- T3 Toxicosis (Elevated T3 with Suppressed TSH and Normal T4)
- Thyroid Hormone Resistance or Exogenous Liothyronine Administration
Decreased (Low) Values
- Euthyroid Sick Syndrome (Non-Thyroidal Illness Syndrome / Low T3 Syndrome)
- Hypothyroidism (Primary or Secondary)
- Severe Malnutrition, Starvation, or Systemic Catabolic State
Clinical Alignment & Significance
Elevated FT3 with suppressed TSH indicates hyperthyroidism or T3 toxicosis; in severe systemic non-thyroidal illness, low FT3 reflects impaired peripheral T4-to-T3 5'-deiodination (Euthyroid Sick Syndrome) rather than primary thyroid failure.
Diagnostic Guidance Notice
Standard laboratory reference ranges vary based on the testing facility. Significant deviations require immediate clinician review to rule out severe medical pathology.
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Reference Citations & Evidence Sources
Clinical Guidelines & Consensus Statements
Clinical Reviews & Textbooks
- CIT-0023National Center for Complementary and Integrative Health. "Homeopathy: What You Need To Know." National Institutes of Health (2021).
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L0010Dr. Narayan Jethwani. "Triiodothyronine (T3 / FT3)." Homeo Healthcare Clinical Platform. Version 1.1.0. Reviewed: 2026-07-31T19:50:00Z. Available at: https://homeo.healthcare/knowledge/lab-tests/t3
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