Triiodothyronine (T3) is made by the thyroid and uses Selenium, Zinc and Iodine to do so.
T3 is also made by the liver by converting T4.
TSH from the Pituitary Gland causes the thyroid gland to make two hormones: triiodothyronine (T3) and thyroxine (T4).
Testing the blood for T3 levels is a good indicator if the thyroid is working correctly or not, however if there is a deficiency of Selenium, Zinc and Iodine, a low level of T3 may not be an indicator of a thyroid not working, but more an indicator of the deficiency. This is why it is important to also test for other indicators.
eg A normal level of T4 combined with a low T3 may not be an indicator of problem thyroid function, because if the thyroid can’t convert the T4 to T3, due to a mineral deficiency, then the problem is the deficiency, not the thyroid.
Synthetic thyroxin such as Synthoid is T4, so if you are taking it, the T4 levels may show as correct even with low T3.
Natural forms of desiccated thyroxin will normally contain a mix of both T4 and some T3.