Informing & Supporting Thyroid Patients Since 1985
Informing & Supporting Thyroid Patients Since 1985
Thyroid Research 
Thyroid Research Archive Thyroid Horme Therapy
(March 2005)
The background of the study. The efficacy of thyroxine (T4) therapy in patients with hypothyroidism depends on several factors, including body weight. In this study the effect of variations in body composition on the dose of T4 was determined in patients with thyroid carcinoma.
How the study was done. Serum thyrotropin (TSH) and free T4 were measured in 75 patients with thyroid carcinoma who had taken the same dose of T4 for at least six months. They were subdivided into three groups according to their body weight: normal weight, overweight, and obese. All had been treated with thyroidectomy and radioactive iodine, and all had undetectable serum thyroglobulin concentrations. All had low serum TSH concentrations (0.005 to 0.3 mU/L) and normal serum free T4 concentrations. Total and peripheral lean and adipose tissue mass was measured by absorptiometry.
The results of the study. The doses of T4, expressed either as the total daily dose or as micrograms per kilogram of body weight per day, were higher in the overweight and obese groups than in the normal-weight group, but the serum TSH and free T4 concentrations in the three groups were similar.
The daily dose of T4 was higher in the men than in the women (160 vs. 130 µg/day), but not different when corrected for weight (1.8 vs. 1.9 µg/kg/day). The dose of T4 was correlated with body weight, more so with lean body mass, and less with fat mass. Age and daily dose of T4 were negatively correlated, but not after correction for lean body mass.
The conclusions of the study. The daily dose of T4 needed to cause subclinical hyperthyroidism in patients with thyroid carcinoma is determined more by lean body mass, especially peripheral lean body mass, than body weight, fat mass, age, or sex.
The original article. Santini F, Pinchera A, Marsili A, Ceccarini G, Castagna MG, Valeriano R, Giannetti M, Taddei D, Centoni R, Scartabelli G, Rago T, Mammoli C, Elisei R, Vitti P. Lean body mass is a major determinant of levothyroxine dosage in the treatment of thyroid diseases. J Clin Endocrinol Metab 2005;90:124-7.
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