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Endocrine Care: Ferruccio Santini, Aldo Pinchera, Alessandro Marsili, Giovanni Ceccarini, Maria Grazia Castagna, Rocco Valeriano, Monica Giannetti, Donatella Taddei, Roberta Centoni, Giovanna Scartabelli, Teresa Rago, Claudia Mammoli, Rossella Elisei, and Paolo Vitti Lean Body Mass Is a Major Determinant of Levothyroxine Dosage in the Treatment of Thyroid Diseases J Clin Endocrinol Metab 2005; 90: 124-127 [Abstract] [Full text] [PDF]

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Lean body mass determines thyroxine replacement dose

Uriel S barzel, Uriel S. Barzel   (19 December 2007)

Lean body mass determines thyroxine replacement dose 19 December 2007
Uriel S barzel, Professor of Medicine montefiore Medical Center and Albert Einstein College of Medicine, Bronx NY, Uriel S. Barzel Send letter to journal: Re: Lean body mass determines thyroxine replacement dose uri{at}barzel.org Uriel S barzel, et al.	In their paper on thyroid hormone replacement in central hypothyroidism (1), the authors used body weight to determine the experimental dose of the hormone, using “empirical” levothyroxine (T4) dosage of 1.0 ± 0.05 µg/day/Kg body weight and “body weight adapted” dosage of 1.6 ± 0.05 µg/day/Kg body weight. In their study, they fail to account for the age of their patients or their lean body mass. In 1982, we reported (2) that the T4 replacement dose decreases with age. We showed by multiple regression analysis that age and weight had each a highly significant independent effect on T4 dosage. In that study, patients older than 65 years of age required a T4 dose of 1.86 ± 0.08 µg/day/kg body weight (mean ± SEM) and younger subjects required 2.06 ± 0.10 µg/day/kg body weight. We noted that dosage requirement actually decreased continuously with age in the ambulatory adult patient population. Since lean body mass decreases with age, we further analyzed our data (3) and demonstrated that lean body mass (LBM) was actually the determining factor for the T4 requirement. Based on 75 patients, 24 to 82 years of age, we derived the following formula: T4 (µg/day) = 3.4 x LBM – 11. The regression of T4 on LBM has an intercept near zero with a slope that is 2.5 times greater than that for T4 on age. Our finding was subsequently confirmed by Ferruccio et al (4). We concluded that direct measurement of lean body mass would allow optimization of thyroid hormone replacement in patients with pituitary disease and secondary hypothyroidism. References 1. Slawik M, Klawitter B, Meiser E et al 2007 Thyroid hormone replacement for central hypothyroidism: A randomized controlled trial comparing two doses of thyroxine (T4) with a combination of T4 and triiodothyronine. J Clin Endo Metab 92:4115-4122 2. Rosenbaum RL, Barzel US 1982 Levo-thyroxine replacement dose for primary hypothyroidism decreases with age. Ann Intern Med 96:53-55 3. Cunningham JJ, Barzel US 1984 Lean body mass is a predictor of the daily requirement for thyroid hormone in older men and women. J Am Geriatrics Soc 32:204-207 4. Ferruccio S, Pinchera A, Marsili A et al 2005 Lean body mass Is a major determinant of levothyroxine dosage in the treatment of thyroid diseases. J Clin Endo Metab 90:124-127