Free T4 (Thyroxine): The fraction of T4 that is unbound and available for the body to utilize.
Produced by: Thyroid Gland
Purpose: T4 is not really a hormone but a prohormone for T3. T4’s main purpose is to be available for conversion into T3.
Lab Reference Range: 0.8-1.8 ng/dL
When to order:
Half-life: 5-7 days.
Free T4 is more important than Total T4 because it is a measure of what is available for your body to use.
Thyroxine-binding globulin (TBG) is a protein made in the liver that binds and carries T4 and T3 in the bloodstream.
High Free T4 may mean that you are on too much thyroid medication, or if you are not on thyroid medication it may represent Grave’s Disease.
The thyroid gland produces T4 and T3 in a ratio of 14:1 to 20:1 (so, a lot more T4).
If you think of a fresh water lake’s watershed, the lake is T3 and the watershed is T4. T3 for the most part depends on it’s watershed (T4) for production of T3.
The prescriptions Synthroid, Levothyroid, Levothyroixine are pure T4.
Free T4 is converted to T3 in five main places: Liver, Gut, Muscle, Brain, Thyroid gland.
If you are not on thyroid medication and you have a low Free T4 with Low or normal TSH, you may have secondary hypothyroidism. This is when the pituitary is not functioning properly and not producing TSH in response to a low T4.
Causes of secondary hypothyroidism: