For patients with Hashimoto's thyroiditis, is there a commercially available blood test for detecting abnormalities in the type 1 deiodinase enzyme in order to identify patients who would potentially benefit from T4 and T3 combination therapy?  

Most clinicians decide to use combination therapy based on a weak response to levothyroxine, with patients still complaining of symptoms related to hypothyroidism. The TSH should not be low before selecting dual replacement. 

From a clinical standpoint, there is almost never a need for adding T3 to the therapeutic regimen of patients being treated for hypothyroidism. The exogenously provided T4 gets converted to T3 in the body. Why pay the drug companies a lot of money for something that your body will do for free? The T...