T3 stands for triiodothyronine. The blood test for T3 is the best test to assess thyroid function, as discussed below.
Out of all of the thyroid hormones, T3 is the most active hormone. It exerts its effects on almost every organ in the body, in particular the heart, brain, muscles, bones, skin, intestines and reproductive organs. Moreover, it regulates our metabolism and weight.
Unfortunately, most physicians are under the mistaken impression that T4 is the active thyroid hormone. Sad but true! That’s why they prescribe Levothyroxine, a synthetic form of T4, to treat every patient with hypothyroidism.
Normal Daily Production
Daily total production of T3 is about 32 micrograms (mcg), about 75-80% (24-26 mcg) of which comes from T4 to T3 conversion in the peripheral tissues such as liver, kidney, and muscle. However, about 20-25% (6-8 micrograms) of the total daily production of T3 comes directly from the thyroid gland.
Low T3 Due to Impaired Conversion
It is important to understand that T4 to T3 conversion is under two different enzyme system: Type 2 DI (5′-deiodinase) is in the brain including the pituitary gland whereas Type 1 DI (5′-deiodinase) is in the rest of the body, especially in the thyroid, liver, kidneys and muscles.
Many factors can disrupt the normal functioning of the Type 1 DI in the body. For example, Hashimoto’s Thyroiditis is a clinical disorder which can disrupt Type 1 DI. Subsequently, T4 does not convert appropriately. Unfortunately, physicians often treat hypothyroidism due to Hashimoto’s Thyroiditis with T4, Levothyroxine. Consequently, T3 level in the body remains low which gives rise to symptoms of low Thyroid – hypothyroidism.
On the other hand, Type 2 DI, which is present in pituitary gland, continues to function normally. Therefore, T4 to T3 conversion inside the pituitary gland remains normal. A normal T3 inside the pituitary gland gives rise to a normal level of TSH, a blood test most physicians heavily rely on to monitor hypothyroid patients on Levothyroxine. Hence, physicians get confused. They seriously believe their patient cannot have symptoms of low thyroid if TSH is in the normal range.
T3 – The Best Test to Assess Thyroid Function
The blood test – Free T3 – is the best test to assess the function of thyroid gland because Free T3 is the active thyroid hormone. On the other hand, T4 test measures the reservoir of thyroid hormone. The other thyroid test – TSH – is an indirect way to assess thyroid function and can be a misleading test.
Excerpts from my book, “Hypothyroidism And Hashimoto’s Thyroiditis”
