Free Thyroxine

Measures the free fraction of thyroxine in the blood.

To assess thyroid function and hormone treatment.

1. Check in advance whether the time of day, the day of your menstrual cycle or other timing matters.
2. Tell your doctor about medicines, hormone preparations and supplements. For some hormones, the collection time or cycle day matters.
3. Tell your doctor about hormone preparations and dietary supplements; do not change your treatment on your own.
4. Stay calm before the blood draw and keep to the time agreed with you.

Fasting is often not necessary for this test alone. If other tests have also been ordered, or your doctor has prescribed specific preparation, follow the agreed rules.

A specific time is generally not required. If your doctor has set a different time, follow those instructions.

• Hormone preparations and supplements
• Blood draw time or cycle phase, when relevant
• Immunoassay interference, including biotin on some platforms

Venous blood — serum or plasma, depending on the method

The blood sample is collected by a qualified staff member. The tube type, required volume, and processing and storage conditions are set by the laboratory's specific method. You do not need to store the sample yourself.

FT4 interpretation depends on TSH and the clinical picture. The meaning of a high or low result depends on age, symptoms, the method used and the reference interval.

If your result is outside the reference interval, you have symptoms, or your condition changes, contact your doctor. If symptoms are severe or develop quickly, seek emergency care.

High-dose biotin can alter some immunoassay results; whether and how long to stop it depends on the dose and the method.