T3 Resin Uptake (T3 Uptake)

Despite its name, the T3 Resin Uptake (T3 Uptake) test does not directly measure T3 but the thyroxine-binding globulin (TBG) capacity to bind to thyroid hormones. This can measure the ratio between thyroid hormones and transport proteins. In this article, I'll discuss the T3 uptake test, how pregnancy affects values and common causes of high and low readings.

T3 Uptake Testing

During the T3 uptake test, a radioactive or nonradioactive thyroxine (T4) analog is added to the blood sample. This analog competes with endogenous T4 in the blood for binding sites on thyroid-binding proteins like TBG and albumin. The percentage of T4 analogs bound to these proteins is then measured. A higher percentage of bound T4 analogs indicates a lower binding capacity of the proteins, while a lower percentage suggests a higher binding capacity.

T3 Uptake in Pregnancy

Estrogen stimulates TBG synthesis. As estrogen levels increase during pregnancy, TBG production also rises, leading to higher circulating levels of TBG-bound thyroid hormones.

Due to this, there is a slight drop in T3 uptake during pregnancy due to this rise. This increase is also the cause of increased total and decreasing decrease of free T4/T3. The percentage should stay within standard ranges.

High T3 Uptake in Pregnancy

There is limited data on the consequences of high T3 uptake in pregnancy.

​An overproduction of thyroid hormones or a decrease in TBG = more thyroid hormones in ratio to thyroid binding proteins. 

Most Common Causes of High T3 Uptake in Pregnancy

• Gestational Transient Thyrotoxicosis (GTT)
• Autoimmune Hyperthyroidism (Grave's Disease)
• Overdose of Medication
• Low Thyroxine-Binding Globulin Production
• Low Estrogen
• Vitamin B6 Deficiency
• Low Protein Diet

Low T3 Uptake in Pregnancy

​There is limited data on the consequences of low T3 uptake in pregnancy.

​Fewer thyroid hormones or an overproduction of TBG = fewer thyroid hormones in ratio to thyroid binding proteins.

Most Common Causes of Low T3 Uptake in Pregnancy

• Hypothyroidism
• Low Total Thyroxine (T4)​

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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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