Vitamin A is required for T4-to-T3 conversion. If you find that lights are too bright or night driving is a problem, try taking Vitamin A supplements along with more protein. Zinc, selenium, and magnesium are also involved in T4-to-T3 conversion.
New Vitamin B12 Test - Who should get tested for vitamin B12 (cobalamin) deficiency and which is the best test to use: serum B12, methylmalonic acid (MMA), or holotranscobalamin levels? - By Dr. Michael Greger