The widely accepted recommendation for monotherapy with L-thyroxine and against the use of products containing T3 is not cookie cutter medicine. Neither is it some guideline writer's opinion. Rather, it's in conformity with best evidence, and the evidence is extensive. The most definitive literature review is here. It concludes:
Conclusions: Until clear advantages of levothyroxine plus liothyronine are demonstrated, the administration of levothyroxine alone should remain the treatment of choice for replacement therapy of hypothyroidism.
There has been nothing additional published since that paper to indicate benefits of the use of T3. Careful analysis of the published studies indicates that those patients who indicated they felt better with the addition of T3 tended to lose weight and be over-replaced. Over-replacement is bad because it increases the risk of atrial fibrillation and osteoporosis. In at least one study the subjective benefits were transient. That makes perfect sense. T4, the inactive prohormone, is only very gradually converted to T3. The gradual onset of effect may blunt subjective awareness. T3 given directly is rapidly absorbed and its relatively short half life results in an initial “buzz” with maximal effect achieved in days as opposed to weeks. As far as I know there are no comparative studies which followed patients long enough to assess how the potentially devastating effects of osteoporosis, atrial fibrillation and loss of muscle mass (all consequences of over-replacement) ultimately impacted quality of life!
It is claimed in the video that some patients lack the ability to convert T4 to T3. That claim is unsupported as far as I know. A related claim is that TSH monitoring is not enough to assess the adequacy of therapy. I won't belabor the evidence and rationale here except to say that God and the patient's pituitary (assuming the patient has primary rather than secondary hypothyroidism) know better than anyone else how much and what kind of replacement the patient needs.
There may be occasional patients, rare exceptions, who benefit from combined therapy. If this is done it should be with the appropriate expertise and great caution against the hazards of osteoporosis and atrial fibrillation. Unfortunately, though, it sometimes falls into the realm of quackery.
Like Diana Hsieh I am concerned about a coming era of cookie cutter medicine, but this was a bad example. Her policy points are great. Unfortunately the bad science used in this case to back them up weakened the credibility. We can have evidence based medicine without cookie cutter medicine.