Thyroxine (T4), Total

Thyroxine (T4) is the primary thyroid hormone secreted by the thyroid gland. It is often measured as total or free. In this article, we will discuss the total T4 test, how it can be used in pregnancy, and why it may be more accurate at detecting thyroid dysfunction than the more commonly run free T4. 

Total T4 Testing

The Total Thyroxine (T4) test measures the total amount of T4 in the bloodstream. 

Total T4 = Protein-Bound T4 + Free T4

Total T4 in Pregnancy

Total T4 production increases by 50% during the first half of pregnancy, plateauing around 20 weeks gestation.  This increase is caused by an increase in thyroxine production as well as an increase in the production of thyroxine-binding globulins (TGB). This results in a higher value of total T4 with a lower free T4. 

While total T4 levels may fluctuate postpartum, the levels of free T4 are typically maintained within a more stable range.

This makes the Total T4 a more accurate measure of thyroid hormone production during pregnancy, and free T4 more accurate during postpartum.

It is always advised that total T4 and free T4 be run together for a comprehensive analysis of thyroid function.

​According to the Endocrine Society, European Thyroid Association, and American Thyroid Association, the use of total T4 to assess thyroid function is more accurate than free T4 in pregnancy. 

​An article published by the American Journal of Obstetrics & Gynecology in 2014 concluded that total T4 and free T4 were equally as accurate in diagnosing thyroid disease in pregnancy.

High Total T4 in Pregnancy

​There is limited data on the consequences of high total T4 in pregnancy. (See TSH Article)

Most Common Causes of High Total T4 in Pregnancy

• Gestational Transient Thyrotoxicosis (GTT)
• Autoimmune Hyperthyroidism (Grave's Disease)
• Medication Overdose
• Thyroid Nodules
• Pituitary Disease

Low Total T4 in Pregnancy

Hypothyroxinemia occurs in around 85% of preterm babies.
​
Evidence suggests that maternal hypothyroxinemia alone poses an increased risk for fetal neurodevelopmental deficits.

Most Common Causes of Low Total T4 in Pregnancy

• Autoimmune Thyroid Disease (Hashimoto's)
• Iodine Deficiency
• Iron Deficiency
• Other Nutritional Deficiencies (vitamin A, vitamin D, magnesium, zinc, selenium)
• Environmental Toxin Exposure
• Heavy Metal Exposure
• Low Progesterone
• Low Protein Diet

Want to Learn More?

​Explore my Advanced Maternal Lab Analysis course—MEAC-approved for continuing education.

Start Your Journey Here

---

Copyright © 2024 Functional Maternity, all rights reserved.

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.​

---