It is often suggested and argued that T3 replacement is required by at least some hypothyroid people - and that T4 alone does not work for them. Now there is some evidence that this is absolutely the case, and it even identifies a genetic polymorphism that is associated with this.
Although published in May, it seems only now to have made it into PubMed. Unfortunately, so far only the abstract appears to be available.
F1000 Med Rep. 2010 May 11;2. pii: 34.
New insights into thyroid hormone replacement therapy.
Acosta BM, Bianco AC.
Division of Endocrinology, Diabetes and Metabolism, University of Miami Miller School of Medicine 1450 NW 10 Avenue #3054, Miami, FL 33136 USA.
It is widely accepted that thyroid hormone replacement for patients with hypothyroidism can be fully accomplished with levothyroxine monotherapy, as assessed by serum thyroid function tests. However, approximately 10% of hypothyroid patients are dissatisfied with the outcome of levothyroxine monotherapy, and physicians continue to report benefits from combined levothyroxine-triidothyronine therapy for some hypothyroid patients. Recently, a large prospective study reported that the benefit of the combined levothyroxine-triidothyronine therapy is associated with the Thr92Ala polymorphism in the type 2 deiodinase gene, which is present in about 15% of the general population. If confirmed, these findings indicate that personalized medicine is rapidly catching up with modern thyroidology.