Unit Converter
Triiodothyronine (T3)
(Active Thyroid Hormone - Sensitive Marker of Thyroid Function & Metabolic Status)
Synonyms
- Total T3
- Serum T3
- Triiodothyronine
- TT3
- 3,5,3′-triiodothyronine
- Bound + free T3
Units of Measurement
- nmol/L
- ng/mL
- ng/dL
- ng/100 mL
- ng%
- ng/L
- µg/L
Unit Conversions
Molecular Weight of T3 = 650.97 g/mol
nmol/L ↔ ng/mL
1 nmol/L=0.651 ng/mL1\ \text{nmol/L} = 0.651\ \text{ng/mL}1 nmol/L=0.651 ng/mL 1 ng/mL=1.54 nmol/L1\ \text{ng/mL} = 1.54\ \text{nmol/L}1 ng/mL=1.54 nmol/L
nmol/L ↔ ng/dL
1 nmol/L=65.1 ng/dL1\ \text{nmol/L} = 65.1\ \text{ng/dL}1 nmol/L=65.1 ng/dL 1 ng/dL=0.0154 nmol/L1\ \text{ng/dL} = 0.0154\ \text{nmol/L}1 ng/dL=0.0154 nmol/L
ng/mL ↔ ng/L / µg/L
1 ng/mL=1000 ng/L=1 µg/L1\ \text{ng/mL} = 1000\ \text{ng/L} = 1\ \text{µg/L}1 ng/mL=1000 ng/L=1 µg/L
ng/dL = ng/100 mL = ng%
(Directly equivalent older units.)
Description
Triiodothyronine (T3) is the biologically active thyroid hormone, responsible for most metabolic effects of thyroid function.
Sources:
- 20% produced directly by the thyroid gland
- 80% produced by peripheral conversion of T4 → T3 via deiodinases
T3 exists in two fractions:
- Bound T3 (~99.7%) → to TBG, albumin, transthyretin
- Free T3 (~0.3%) → active form
Total T3 is influenced by binding proteins but remains clinically important in hyperthyroidism evaluation.
Physiological Role
T3 regulates:
- Basal metabolic rate
- Heat production
- Heart rate & contractility
- Lipid and carbohydrate metabolism
- GI motility
- Bone turnover
- Brain development (fetal & infant)
- Mood and cognition
T3 is 3–4 times more potent than T4.
Clinical Significance
HIGH T3
1. Graves’ Disease
Often shows disproportionately high T3 (“T3 toxicosis”).
2. Toxic Multinodular Goiter
3. Toxic Adenoma
4. Thyroiditis
5. Excess Thyroid Hormone Intake
- Over-replacement
- Factitious hyperthyroidism
6. TSH-secreting pituitary adenoma
High T3 with high TSH.
7. Thyroid Hormone Resistance
High T3 with normal/high TSH.
Symptoms
- Palpitations
- Tremors
- Weight loss
- Heat intolerance
- Anxiety
- Tachycardia / AF
LOW T3 (Hypotriiodothyroninemia)
1. Non-thyroidal Illness Syndrome
Most common cause of low T3:
Critical illness, sepsis, trauma, liver/kidney failure.
T3 decreases early due to reduced T4→T3 conversion.
2. Hypothyroidism
Typically FT4 gives better diagnostic accuracy.
3. Starvation / Malnutrition
Body lowers metabolic rate.
4. Liver Disease
Reduced T4→T3 conversion.
5. Medications
- Amiodarone
- Beta blockers (propranolol)
- Glucocorticoids
- Lithium
Reference Intervals
(Tietz 8E + ATA + Mayo + ARUP)
Total T3
- 1.2 – 2.8 nmol/L
(= 80 – 180 ng/dL)
(= 0.8 – 1.8 ng/mL)
Critical Interpretations
- T3 > 3.5 nmol/L → strong hyperthyroidism
- T3 < 1.0 nmol/L → possible hypothyroidism or NTIS
Diagnostic Uses
1. Diagnosis of Hyperthyroidism
Especially when T3 is disproportionately high (T3-toxicosis).
2. Differentiating Thyroid Disorders
T3 is a sensitive indicator of active thyroid hormone excess.
3. Non-thyroidal Illness
Low T3 is earliest thyroid change in critical illness.
4. Monitoring Therapy
In special situations:
- T3-containing therapy
- PTU therapy (affects peripheral conversion)
5. Pediatric Growth & Development
Assess thyroid metabolism in infants.
Analytical Notes
- Serum sample preferred.
- Interference:
- Biotin supplementation
- Heterophile antibodies
- High TBG alters total T3 but not free T3.
- Biotin supplementation
- T3 assays vary; free T3 accuracy is limited vs. FT4.
Clinical Pearls
- T3 toxicosis = high T3 with normal T4 → early Graves’.
- NTIS: low T3, normal/low T4, normal/low TSH - DO NOT treat with thyroid hormones.
- Elderly hyperthyroid patients may have normal T3 (“apathetic hyperthyroidism”).
- T3 rises more than T4 in hyperthyroid states.
- Total T3 is more affected by TBG changes than FT3.
Interesting Fact
Although T3 represents only 5–10% of circulating thyroid hormone concentration, it exerts over 90% of biological thyroid hormone activity.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Thyroid Hormones
- ATA / AACE Hyperthyroidism Guidelines
- European Thyroid Association (ETA) Guidelines
- Mayo Clinic Laboratories - Total T3
- ARUP Consult - Thyroid Function
- NIH / MedlinePlus - Triiodothyronine