Triiodothyronine (T3), Total

Total Triiodothyronine (T3) is rarely, if ever, run in maternity care blood work, but it can help to create a more comprehensive overview of maternal thyroid function. In this article, I'll cover total T3 testing in pregnancy, as well as common causes of both high and low values. 

Total T3 Testing

The Total Triiodothyronine (T3)  test measures the total amount of T3 in the bloodstream.

Total T3 = Protein-Bound T3 + Free T3

Only 20% of circulating T3 comes directly from the thyroid gland, and>80% is produced by the liver and other tissues via deiodinase enzyme breakdown of T4. 

Total T3 in Pregnancy

Total T3 increases gradually and steadily throughout pregnancy, peaking in the third trimester. ​During the postpartum phase, there is a period of minor hypothyroidism that should correct itself in the first few weeks of postpartum. Total T3 levels should return to normal between four to six weeks postpartum.

High Total T3 in Pregnancy

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

Most Common Causes of High Total T3 in Pregnancy

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

Low Total T3 in Pregnancy

​Low T3 levels have been shown to negatively affect fetal brain development, particularly the hippocampus.

Most Common Causes of Low Total T3 in Pregnancy

• T3 Syndrome
• Oxidative Stress
• High Reverse T3
• Autoimmune Thyroid Disease (Hashimoto's)
• Nutritional Deficiencies (vitamin A, magnesium, selenium)

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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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