Free Thyroxine Index (FTI/FT4I)

The Free Thyroxine Index (FTI/FT4I) test use to be the standard test for assessing thyroid hormone levels. Today, the free T4 test is used in its place. A few thyroid panels still add this chemistry marker. In pregnancy, the FTI may be more accurate to assess thyroid hormone levels due to hormone changes that can affect other thyroid indices. In this article, I'll discuss the current research surrounding the use of the FTI in pregnancy.

FTI Testing

The Free Thyroxine Index (FTI) estimates the actual free T4. 

​FTI = Total T4/T3 Uptake × 100

The direct measurement of FT4 is considered a more accurate assessment of the biologically active form of thyroxine. Based on the hormonal changes in pregnancy that affect the total and free T4 levels, some researchers suggest that FTI is a more accurate measure to assess FT4 in pregnancy.

ACOG and other organizations have stated that FTI is a better assessment of true availability of FT4, yet do not offer a trimester-specific reference range for guidance. 

FTI in Pregnancy

​FTI increases in conjunction with total and free T4. TT4 and FTI should maintain an inverse relationship to TSH during pregnancy. 

​According to a 2002 American College of Obstetricians and Gynecologists bulletin on thyroid disease, TSH and FT4 or FTI testing should be performed in pregnant women with suspected hyperthyroidism or hypothyroidism.

High FTI in Pregnancy

There is limited data on the consequences of high FTI in pregnancy. ​

Most Common Causes of High FTI in Pregnancy

• Gestational Transient Thyrotoxicosis (GTT)
• Autoimmune Hyperthyroidism (Grave's Disease)
• Stress
• Inflammation
• Overdose of Medication

Low FTI in Pregnancy

There is limited data on the consequences of low FTI in pregnancy. (See TSH Section)

Most Common Causes of Low FTI in Pregnancy

• Autoimmune Hypothyroidism (Hashimoto's)
• Hypothyroidism

Want to Know More About Thyroid Labs in Pregnancy?

​Explore Advanced Maternal Lab Analysis—MEAC-approved for CE credits.

Start Learning Here

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Medical Disclaimer:
This content is for educational purposes only and is intended for licensed medical professionals. It is based on current research, subject to change, and not intended to diagnose, treat, or prevent any medical condition. This information does not replace consultation with a qualified healthcare provider. Seek professional medical evaluation and treatment from a licensed provider. Use of this information is at your own risk.​

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