There is actually a good reason for not screening for fT3/T3. The reason has to do with the accuracy of the assays and what they are telling you. I have this on excellent authority directly from one of the top endocrinologist in the UK (who incidentally is still one of the few who will prescribe T3). Unfortunately what is relevant in terms of fT3/T3 is intracellular levels and the assays can’t accurately measure this. So the proxy for this is your TSH and associated level of fT4 where they know for the vast majority of the population will cause sufficient conversion to intracellular T3. In fact for most on levothyroxin they display low range fT3 levels. Now there are certain populations (i.e. some of those on lithium treatment and some genetic conditions etc.) where there is no to little conversion of T4 to T3 where T3 supplementation is warranted. There are also a subgroup of hypothyroid patients who for whatever reason just do better on T3/T4 combination treatment and then some that just do better on desiccated thyroid replacement and they don’t know why although as I mention above there are other thyroid hormones (calcitonin) - which occur in desiccated thyroid and obviously don’t with synthetic T3/T4 - and for this subgroup these hormones may matter. At the end of the day the endocrinologist should always be willing to provide a trial of T3 on its own or in combination with T4 or with a desiccated thyroid replacement option if the patient remains symptomatic despite adequate replacement levels of T4 and a low therapeutic level of TSH.

Send me a PM if you want to chat more or want a source for desiccated thyroid.