Total Triiodothyronine (T3)

Thyroid

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No Fasting Required

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Checks thyroid function

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Total Triiodothyronine (T3) Test Guide

Why is it done?
- Measures total circulating triiodothyronine (T3), including both protein-bound and free T3 produced by the thyroid gland
- Diagnoses and monitors thyroid disorders such as hyperthyroidism, hypothyroidism, and thyroiditis
- Evaluates symptoms of thyroid dysfunction including fatigue, weight changes, temperature sensitivity, and heart palpitations
- Monitors effectiveness of thyroid hormone replacement therapy or antithyroid medications
- Assesses patients with abnormal TSH levels to differentiate between different causes of thyroid dysfunction
- Investigates cases of suspected secondary hyperthyroidism or T3 toxicosis
- Typically performed during initial thyroid screening, when TSH is abnormal, or during medication adjustment
Normal Range
- Reference Range: 60-180 ng/dL (0.92-2.76 nmol/L) for adults
- Units of Measurement: ng/dL (nanograms per deciliter) or nmol/L (nanomoles per liter); ranges may vary slightly by laboratory
- Normal Results: Indicate adequate thyroid function with appropriate hormone levels for maintaining normal metabolism
- High Results (>180 ng/dL): Suggest hyperthyroidism or elevated thyroid hormone levels; may indicate overtreatment with thyroid medication
- Low Results (<60 ng/dL): Suggest hypothyroidism or insufficient thyroid hormone; may indicate inadequate treatment or thyroid dysfunction
- Age-Specific Variations: Newborns and infants may have higher reference ranges; reference ranges may vary with age and sex
Interpretation
- Elevated T3 (>180 ng/dL): May indicate Graves' disease, toxic multinodular goiter, thyroiditis, or thyroid hormone overreplacement; requires correlation with TSH and free T4 levels
- Decreased T3 (<60 ng/dL): Suggests hypothyroidism, primary thyroid disease, secondary hypothyroidism, or inadequate thyroid hormone replacement; may occur with certain medications or severe illness
- T3 Toxicosis: Condition where T3 is elevated but T4 is normal or low; suggests selective T3 overproduction and requires specific clinical investigation
- Factors Affecting Results: Medications (estrogen, lithium, amiodarone), pregnancy, severe illness, fasting/malnutrition, immune disorders, and recent radioactive iodine treatment
- Clinical Significance with Other Tests: Results must be interpreted together with TSH and free T4; discordant patterns help diagnose specific thyroid conditions and medication effects
- Binding Protein Considerations: Total T3 measures both protein-bound and free forms; abnormal binding proteins due to pregnancy, liver disease, or nephrotic syndrome can affect results
Associated Organs
- Primary Organs Involved: Thyroid gland (produces T3), hypothalamus and pituitary gland (regulate thyroid function via TSH), and liver (metabolizes thyroid hormones)
- Conditions Associated with Abnormal T3: Graves' disease, Hashimoto's thyroiditis, toxic nodular goiter, thyroid cancer, thyroiditis, primary hypothyroidism, secondary hypothyroidism, and pituitary disorders
- Diseases Diagnosed or Monitored: Hyperthyroidism, hypothyroidism, thyroid cancer, thyroiditis, autoimmune thyroid disease, and metabolic syndrome; also helps evaluate systemic conditions affecting thyroid function
- Potential Complications of Abnormal T3: Elevated T3 may cause atrial fibrillation, osteoporosis, anxiety, and thyroid storm; low T3 may cause severe fatigue, depression, weight gain, hypothermia, and myxedema coma
- Metabolic Effects: T3 is the most metabolically active thyroid hormone, affecting heart rate, body temperature, energy expenditure, and multiple organ systems including cardiovascular, neurological, and gastrointestinal systems
Follow-up Tests
- Routine Follow-up Tests: TSH (Thyroid Stimulating Hormone) and Free T4 to establish complete thyroid function profile and differentiate between primary and secondary thyroid disorders
- Additional Investigations: Free T3 (if T3 toxicosis suspected), thyroid antibodies (TPO, thyroglobulin) for autoimmune disease, thyroid ultrasound, and radioiodine uptake scan for structural or functional abnormalities
- Monitoring Frequency: Every 6-8 weeks after starting or adjusting thyroid medication; every 6-12 months for stable patients on replacement therapy; more frequently during acute illness or medication changes
- Complementary Tests: Reverse T3, thyroglobulin (for cancer monitoring), liver function tests, and basic metabolic panel to assess metabolic effects and medication metabolism
- Imaging Studies: Thyroid ultrasound or nuclear imaging if structural abnormalities are suspected or if T3 levels remain unexplained
- Repeat Testing: Should be performed 6-8 weeks after initiating or changing thyroid medication to assess adequacy of dosage; also when symptoms change or compliance is questioned
Fasting Required?
- Fasting Requirement: No, fasting is not required for Total T3 testing
- Blood Draw Timing: Can be performed at any time of day; however, early morning collection is preferred for consistency in serial measurements due to diurnal variations in hormone levels
- Medications to Avoid: Do not discontinue thyroid medications before testing unless specifically instructed by physician; biotin supplements should be discontinued 2 days prior to testing as they may interfere with assay accuracy
- Special Instructions: Drink water normally; maintain regular sleep schedule; avoid unusual physical stress 24 hours before testing; inform healthcare provider of all medications including supplements and recent contrast media exposure
- Patient Preparation: Allow 5-10 minutes rest before blood draw; remove blood pressure cuff from testing arm; wear comfortable, loose-fitting clothing for easy blood draw access
- Factors Affecting Accuracy: Certain medications (estrogen, lithium, amiodarone, corticosteroids), iodine-containing products, and radioiodine administration may affect results; notify the laboratory of these factors

How our test process works!