Triiodothyronine (T3), Free

Triiodothyronine (T3) is most commonly run to diagnose hyperthyroidism and is rarely tested in maternity care. It is an important piece of the total thyroid health picture and can be used to assess overall function in pregnancy. In this article, I'll discuss the free T3 test, how pregnancy affects the accuracy of lab values, and what high and low values may mean in maternity health. 

Free T3 Testing

The Free Triiodothyronine (T3) test measures only the free T3 in the blood sample. 

​Only 0.3% of T3 is free, yet free T3 is not influenced by variations in protein binding (e.g., TBG and albumin levels), and measuring free T3 provides a more accurate representation of the biologically available and active thyroid hormone levels.

While T4 primarily binds with thyroid-binding globulin (TBG), T3 has a higher affinity for albumin.

Free T3 in Pregnancy

Free T3 increases in the first trimester and decreases through the second and third trimesters. Free T3 can decrease in the first few weeks postpartum as the thyroid recovers from pregnancy. 

High Free T3 in Pregnancy

​There is limited data on the consequences of high free T3 in pregnancy. (See TSH Article)

Most Common Causes of High Free T3 in Pregnancy

• Gestational Transient Thyrotoxicosis (GTT)
• Autoimmune Thyroid Disease (Grave's Disease)
• Low Albumin
• Low Protein Diet
• Thyroid Hormone Resistance Syndrome
• Oxidative Stress
• Overdose of Medication
• Insulin Resistance 

Low Free T3 in Pregnancy

​There is limited data on the consequences of low free T3 in pregnancy. (See Total Triiodothyronine (T3) Article)

Most Common Causes of Low Free T3 in Pregnancy

• T3 Syndrome
• Nutritional Deficiencies (magnesium, selenium, zinc)
• Progesterone Deficiency

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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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