Free Thyroxine (FT4)

Thyroid

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No Fasting Required

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Evaluates Free T4 (Thyroxine), which helps assess thyroid gland function and metabolism.

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Free Thyroxine (FT4) Test Information Guide

Why is it done?
- Measures the amount of unbound thyroxine hormone circulating in the blood, representing the biologically active form of thyroid hormone available to body tissues
- Diagnoses thyroid disorders including hypothyroidism, hyperthyroidism, and thyroid dysfunction
- Monitors thyroid hormone replacement therapy in patients taking levothyroxine or other thyroid medications
- Evaluates symptoms such as fatigue, weight changes, temperature sensitivity, hair loss, and mood disturbances
- Screens for thyroid disorders in pregnant women, newborns, and patients with family history of thyroid disease
- Assesses thyroid function in patients taking medications that affect thyroid hormone metabolism
- Typically ordered during initial evaluation of suspected thyroid disease and at regular intervals during treatment monitoring
Normal Range
- Standard Reference Range: 0.8 to 1.8 ng/dL (10.4 to 23.2 pmol/L)
- Units of Measurement: ng/dL (nanograms per deciliter) or pmol/L (picomoles per liter)
- Note: Reference ranges may vary between laboratories and different testing methods. Always compare results to the specific laboratory's reference range provided on the report.
- Normal (Euthyroid): FT4 within the laboratory's reference range indicates proper thyroid function with adequate thyroid hormone production and availability to tissues
- High (Above Reference Range): Elevated FT4 exceeding the upper limit suggests hyperthyroidism or excessive thyroid hormone production
- Low (Below Reference Range): Reduced FT4 below the lower limit indicates hypothyroidism or insufficient thyroid hormone production
- Borderline Values: Results near the upper or lower limits may warrant repeat testing or additional thyroid function tests for accurate diagnosis
Interpretation
- High FT4 Results: May indicate Graves' disease, toxic multinodular goiter, thyroiditis, or excessive thyroid hormone replacement therapy; associated symptoms include weight loss, anxiety, palpitations, tremor, and heat intolerance
- Low FT4 Results: May indicate primary hypothyroidism (Hashimoto's thyroiditis, iodine deficiency, thyroid surgery), secondary hypothyroidism (pituitary dysfunction), or inadequate thyroid hormone replacement; associated symptoms include fatigue, weight gain, cold intolerance, constipation, and depression
- FT4 with TSH Correlation: High FT4 with low TSH indicates primary hyperthyroidism; low FT4 with high TSH indicates primary hypothyroidism; discordant results may suggest secondary thyroid disorders or pituitary dysfunction
- Pregnancy Considerations: Trimester-specific reference ranges exist; FT4 changes during pregnancy affect fetal development; abnormal results warrant close monitoring and possible treatment adjustment
- Medication Effects: Levothyroxine, amiodarone, beta-blockers, estrogen, and other medications can affect FT4 levels; timing of blood draw relative to medication administration affects results
- Clinical Context: Interpretation requires consideration of clinical symptoms, TSH levels, patient age, sex, pregnancy status, medications, and underlying medical conditions for accurate diagnosis
Associated Organs
- Primary Organs: Thyroid gland (produces thyroxine); Pituitary gland (produces TSH that regulates thyroid); Hypothalamus (produces TRH that regulates pituitary)
- Conditions Associated with High FT4: Graves' disease (autoimmune hyperthyroidism), Plummer's disease (toxic multinodular goiter), thyroiditis (acute inflammation), thyroid cancer, excessive thyroid hormone replacement, iodine-induced hyperthyroidism
- Conditions Associated with Low FT4: Hashimoto's thyroiditis (autoimmune hypothyroidism), thyroid surgery or radioactive iodine ablation, iodine deficiency, central hypothyroidism (pituitary/hypothalamic dysfunction), medications interfering with thyroid function
- Complications of Abnormal FT4: Hyperthyroidism complications include cardiac arrhythmias, heart failure, thyroid storm, bone loss, and ophthalmologic manifestations; hypothyroidism complications include myocardial infarction, cognitive decline, growth impairment in children, and coma in severe cases
- Systemic Effects: Thyroid hormone affects metabolism, cardiovascular function, neurological development, reproductive health, bone metabolism, and virtually all body systems through effects on growth, temperature regulation, and energy expenditure
Follow-up Tests
- TSH (Thyroid-Stimulating Hormone): Routinely ordered alongside FT4 to evaluate pituitary-thyroid axis; helps differentiate primary from secondary thyroid disorders
- Free Triiodothyronine (FT3): Ordered when hyperthyroidism is suspected or to evaluate T3 thyroiditis; more specific for certain hyperthyroid conditions
- Thyroid Peroxidase (TPO) Antibodies: Ordered to diagnose autoimmune thyroiditis (Hashimoto's) when hypothyroidism is confirmed
- Thyroglobulin Antibodies: Used to identify autoimmune thyroid disease alongside TPO antibodies
- TSH Receptor Antibodies (TRAb): Ordered when Graves' disease is suspected to confirm diagnosis and assess disease activity
- Thyroid Ultrasound: Imaging study to evaluate thyroid structure, detect nodules, or assess gland size when abnormal FT4 is detected
- Repeat FT4 Testing: Recommended 6-8 weeks after initiating or adjusting thyroid hormone replacement therapy; ongoing monitoring at 6-12 month intervals for stable patients; more frequent testing during initial titration phase
- Radioactive Iodine Uptake (RAIU): Nuclear medicine study ordered when hyperthyroidism etiology needs clarification to differentiate Graves' disease from thyroiditis
- Comprehensive Metabolic Panel: May be ordered to assess for complications of thyroid disease or effects on other organ systems
Fasting Required?
- Fasting Requirement: No - Fasting is NOT required for FT4 testing
- Meal and Fluid Intake: Patient may eat and drink normally; no dietary restrictions apply to this test
- Medication Timing: CRITICAL: Levothyroxine or other thyroid medications should be taken at least 4-6 hours BEFORE blood draw, preferably taken in the morning and blood drawn in the afternoon; avoid taking thyroid medication immediately before testing to prevent artificially elevated results
- Medications to Avoid or Report: Biotin supplements (can interfere with assay - discontinue 7 days before testing); inform healthcare provider about estrogen, amiodarone, beta-blockers, lithium, and other medications affecting thyroid function
- Supplements to Avoid: Iron and calcium supplements should be taken at least 4 hours apart from levothyroxine as they can interfere with absorption; iodine supplements should be reported to healthcare provider
- Timing of Blood Draw: Morning blood draws are preferred; consistent timing between tests improves comparability of results for treatment monitoring
- General Preparation: Wear comfortable clothing for easy blood draw access; stay hydrated; avoid strenuous exercise immediately before testing; inform phlebotomist of any bleeding disorders or medications affecting coagulation

How our test process works!