2012 ETA Guidelines: The Use of L-T4 + L-T3 in the Treatment of Hypothyroidism
Wiersinga, W.M.; Duntas, L.; Fadeyev, V.; Nygaard, B.; Vanderpump, M.P.J. Eur Thyroid J 2012;1:55-71 http://www.karger.com/Article/FullText/339444

It is suggested to start combination therapy in an L-T4/L-T3 dose ratio between 13:1 and 20:1 by weight (L-T4 once daily, and the daily L-T3 dose in two doses).
Currently available combined preparations all have an L-T4/L-T3 dose ratio of less than 13:1, and are not recommended.
Daily production rate of triiodothyronine (T3) for a body surface area of 1.79 m2 is about 29 µg (16 µg/day/m2): 20% (∼6 µg, or 3.3 µg/day/m2) is secreted by the thyroid gland, and 80% (∼23 µg, or 12.7 µg/day/m2) is generated in extrathyroidal tissues by 5′-deiodination of T4
the half-life of T3 is short (about 1 day), and treatment with L-T3 would require several doses per day with wide variation in serum T3 levels along the 24-hour period.
They(ETA Guidelines) recommend much lower L-T3 doses in combination therapy than previously used, thereby enhancing safety.