Total T4 (Thyroxine): One of the two main thyroid hormones.
Produced by: Thyroid Gland
Purpose: T4 is not really a hormone but a prohormone for T3. T4’s main purpose in life is to be available for conversion into T3.
Lab Reference Range: 4.5-12.0 mcg/dL
When to order:
- To monitor thyroid hormone medication usage.
High total 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).
T4 is the reservoir for T3 production.
Synthroid, Levothyroid, Levothyroxine are all T4 products.
If you are not on thyroid medication and you have a low total 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: