Free Triiodothyronine (Free T3 / FT3)
Clinical interpretation, ATA 2026 guidelines, T3 thyrotoxicosis, euthyroid sick syndrome, and metabolic rate assessment for Free T3 (FT3).
Editorial review complete
Independent clinical validation is pending.
Article Editorial Timeline
Intelligent Context-Aware Learning Path
Objective: Assess Free Triiodothyronine status
2 min readPatient Protocols: Fasting and prep instructions
1 min readFT3: Measures active thyroid hormone
4 min readHypothyroidism: Low thyroid hormone production
6 min readHair Fall: May occur in autoimmune thyroid disease
3 min readTSH: Regulates thyroid gland hormone release
5 min readBook Consultation
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
Free Triiodothyronine (Free T3 / FT3): Unbound, biologically active fraction of the metabolically potent thyroid hormone T3, representing ~0.3% of total circulating T3 generated primarily by peripheral 5'-deiodination of T4 in liver, kidney, and muscle tissues.
Adult Reference Interval: 2.3–4.2 pg/mL (3.5–6.5 pmol/L).
Elevated (High) Values
- T3 Thyrotoxicosis (Suppressed TSH with elevated FT3 but normal FT4)
- Graves' Disease, Toxic Multinodular Goiter, Toxic Adenoma
- Exogenous T3 Therapy (Liothyronine), Peripheral Thyroid Hormone Resistance
Decreased (Low) Values
- Euthyroid Sick Syndrome (Non-Thyroidal Illness Syndrome during severe systemic infection, trauma, or starvation)
- Primary or Secondary Hypothyroidism (though FT3 drops later than FT4)
- Severe Malnutrition, Fasting, Glucocorticoid or High-Dose Propranolol Administration
Clinical Alignment & Significance
Elevated FT3 with suppressed TSH (<0.01 mIU/L) confirms active hyperthyroidism/T3 toxicosis; low FT3 in critically ill ICU patients typically reflects non-thyroidal illness syndrome (euthyroid sick syndrome) rather than primary hypothyroidism.
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).
AI & Generative Search Citation Block
L0035Dr. Narayan Jethwani. "Free Triiodothyronine (Free T3 / FT3)." Homeo Healthcare Clinical Platform. Version 1.1.0. Reviewed: 2026-07-31T20:30:00Z. Available at: https://homeo.healthcare/knowledge/lab-tests/ft3
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