I agree with Red_Star and would add a few more likely possibilities: Low Vitamin D, B6, testosterone. A full list can be found in Recommendation 11 on page 13 of the following link.
https://tinyurl.com/y7sfp5r8
Note also that "successful treatment usually entails first correcting any identifiable cause and then systematically reducing T4 excess and gradually increasing T3 levels until RT3 approaches the middle of its range and Free T3 the upper part of its range. "
Treat the underlying cause. Causes of elevated reverse T3 includes illness or infection, inflammation, emotional or physical stress, very low calorie diets, certain medications, diabetes, low iron or vitamin B12, hyperthyroidism.