Q: What are the standard and optimal reference ranges of T4 to T3 conversion, respectively? What could be the most common reasons for low conversion other than cortisol or liver dysfunction?
A: Optimal ranges are:
Optimal T4 reference range: 15-23 pmol/L
Optimal T3 reference range: 5-7 pmol/L
Those ranges are in picomoles per liter. Some other labs may use different measurements, so just make sure that you are looking at the right ones.
The conversion from T4 to T3 can be impeded by numerous factors, including nutrient deficiencies, stress, infections, and impaired liver and gut activity.
Some reasons for low conversion outside of adrenals and liver, include:
Nutrient deficiencies, such as in selenium
Low ferritin
Gene variations in your deiodinase genes that may prevent you from properly converting T4 to T3
Fasting
Heavy metals
More contributing factors are listed in this article by Dr. Wentz.