A specific dosage equivalency exists between T4 and T3 thyroid hormones
Clinical guidelines and pharmacological studies establish that specific dosage equivalencies and ratios exist when substituting or combining T4 (levothyroxine) and T3 (liothyronine) thyroid hormones.
The retrieved literature includes specific papers, such as the European Thyroid Association guidelines and specialized pharmacokinetic/pharmacodynamic crossover trials (e.g., [7]), that explicitly determine dosage equivalency and substitution ratios between T4 and T3 (such as approximately a 3:1 ratio by weight). Therefore, the claim is supported by empirical evidence.
Wilmar M. Wiersinga, Leonidas H. Duntas, Valentin Fadeyev, Birte Nygaard, Mark Vanderpump. 2012 ETA Guidelines: The Use of L-T4 + L-T3 in the Treatment of Hypothyroidism. 2012. https://doi.org/10.1159/000339444
The 2012 ETA guidelines suggest starting combination therapy with an L-T4/L-T3 dose ratio by weight between 13:1 and 20:1.
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Francesco S. Celi, Marina Zemskova, Joyce D. Linderman, Nabeel I. Babar, Monica C. Skarulis, György Csákó, Robert Wesley, Rene Costello, Scott Penzak, Frank Pucino. The pharmacodynamic equivalence of levothyroxine and liothyronine: a randomized, double blind, cross‐over study in thyroidectomized patients. 2010. https://doi.org/10.1111/j.1365-2265.2009.03700.x
A clinical study in thyroidectomized patients demonstrated that the therapeutic substitution of L-T3 for L-T4 was achieved at approximately a 3:1 weight ratio (or roughly 1:3 ratio for T3:T4).
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