In order to determine how many binding sites on thyroid hormone-binding proteins in the blood are free and able to bind with triiodothyronine (T3), a test called T3RU is performed. Instead of measuring free T3 in the blood, this method evaluates how well certain proteins bind this hormone.

Possible causes for a low T3RU count include:

• The most prevalent reason for a low T3RU level is hypothyroidism. When the thyroid gland stops producing adequate thyroid hormones, especially thyroxine (T4), a condition known as hypothyroidism develops.
• Steroids and high doses of aspirin are two drugs that have been linked to suppressed T3RU levels in the body.
• Thyroid hormone levels, notably T3RU, might lower during pregnancy.

Instances, where a high T3RU count is seen, include:

• Overactive thyroid (hyperthyroidism) causes an overabundance of thyroid hormones (including triiodothyronine and thyroxine) to be produced. Increased release of thyroid-binding proteins in response to elevated thyroid hormones has been linked to elevated T3RU levels.
• Pregnancy: Since estrogen synthesis rises during pregnancy, thyroid-binding protein levels may rise as a result. This, in turn, can lead to a rise in T3RU levels.
• Liver disease: Abnormal liver function can influence thyroid hormone metabolism and lead to changes in blood levels of thyroid-binding proteins. As a result, it might lead to a rise in the total number of T3RUs.