Free Thyroxine (T₄)

Introduction

Thyroxine (T₄) is the major hormone produced by the thyroid gland. Thyroid hormone levels may fluctuate due to stress, diet, medications, childbirth, and menopause. Hyperthyroidism (excess thyroid hormones) can cause a fast heart rate, high blood pressure, excess sweating, shaky hands, anxiety, and weight loss. Hypothyroidism (low thyroid hormones) slows down body functions, and causes fatigue, puffy eyes and face, dry hair and skin, constipation, slower heart rate, constantly feeling cold, confusion, and depression.

Pricing

$49

How to order a test

Synonyms

• Free T₄

What is Included?

Measurement of free thyroxine levels in a blood sample by chemiluminescent microparticle immunoassay.

Turnaround Time

1 – 3 business days
The turnaround time is not guaranteed. The average turnaround time is 1 – 3 business days from the date that the sample arrives at the laboratory. Shipping time for the sample is not included. Additional time is required if the case requires confirmatory or reflex testing, or if the sample is insufficient, or if a recollection is required.

Related Documents

Additional Information and Resources

» Facts about thyroid-stimulating hormone (TSH)
» What could be affecting my thyroid health?
» Why is hypothyroidism more common in women?
» What are the links between thyroid health and mental health? ​
» All about Hashimoto’s disease and Graves’ disease

Preparation Before Specimen Collection

None

Specimen Type

Blood

Volume

50 μL in a microtainer

Container

Microtainer (regular blood tube)

Collection Method

This test requires a blood sample from a finger prick. All supplies for sample collection are provided in the kit.

1. First wash and dry hands. Warm hands aid in blood collection.
2. Clean the finger prick site with the alcohol swab and allow to air dry.
3. Use the provided lancet to puncture the skin in one quick, continuous and deliberate stroke.
4. Wipe away the first drop of blood.
5. Massage hand and finger to increase blood flow to the puncture site. Angle arm and hand downwards to facilitate blood collection on the fingertip.
6. Drip blood into the microtainer tube.
7. Dispose of all sharps safely and return sample to the laboratory in the provided prepaid return shipping envelope.

NOTES: Avoid squeezing or ‘milking’ the finger excessively. If more blood is required and blood flow stops, perform a second skin puncture on another finger. Do not touch the fingertip.

Specimen Storage

Maintain specimen at temperatures between 2°C and 30°C during storage and transport.

Specimen Stability

Blood samples can be refrigerated or kept at room temperature for up to 7 days.

Causes for Rejection

• Incorrect or incomplete patient identification
• Incorrect specimen collection
• Inappropriate storage and transport conditions
• Incorrect specimen volume

Purpose

To measure free thyroxine (T₄) levels in a blood sample for the evaluation of thyroid function. 

Limitations

• This report is not intended for use in medico-legal applications. 
• These results should be interpreted in conjunction with other laboratory and clinical information.
• Correct specimen collection and handling is required for optimal assay performance. 
• Interferences from medication or endogenous substances may affect results.
• T₄ levels may also be abnormal in nonthyroidal disease. T₃ levels should also be determined for an accurate diagnosis.

Methodology

Chemiluminescent microparticle immunoassay (Alinity i Free T₄ assay)

Reference Intervals

0.70 – 1.48 ng/dL

This reference range was obtained from the Alinity i Free T4 package insert.