Paper
Sex and age differences in levothyroxine dosage requirement.
Published 2010 · J. Jonklaas
Endocrine practice : official journal of the American College of Endocrinology and the American Association of Clinical Endocrinologists
54
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Abstract
OBJECTIVE To examine the performance of weight-based formulae for estimating the levothyroxine dosage requirement in athyreotic patients and to determine whether formula performance is affected by age, sex, or menstrual status. METHODS In this prospective study, euthyroid study participants aged 18 to 65 years were followed up after total thyroidectomy at 4 time points: 6-8 weeks, 12-16 weeks, 6 months, and 1 year. Patient weight, serum thyrotropin concentration, and levothyroxine dosage required were recorded at each time point. The postoperative starting levothyroxine dosage was 1.7 mcg/kg daily for patients with benign thyroid disease and 2.2 mcg/kg daily for patients with thyroid cancer. Actual body weight was used to calculate the initial dosage. At steady state, adjustments were made in each patient's levothyroxine dosage until the target thyrotropin concentration was reached. The levothyroxine dosage required to achieve this goal was documented. RESULTS Fifty patients were included (37 women, 13 men). Formulae based on actual body weight were accurate in achieving a normal thyrotropin concentration in 48% to 75% of participants. Final dosages to achieve normal thyrotropin values were similar in men (1.43 mcg/kg daily) and menopausal women (1.68 mcg/kg daily), but higher in premenopausal women (2.10 mcg/kg daily). When a formula based on ideal body weight was used, the requirement for menopausal women (2.34 mcg/kg daily) was similar to that of premenopausal women (2.44 mcg/kg daily), but the requirement for men (1.73 mcg/kg daily) remained lower than that observed in both female groups. CONCLUSIONS When actual body weight was used to calculate levothyroxine dosage requirement, premenopausal women appeared to have a greater requirement than either menopausal women or men. When ideal weight was used, the requirement of all women was greater than that of men. Perhaps with formulae using actual weight, this apparent sex difference is masked by the greater weight, older age, or altered hormonal milieu of menopausal women.
Observational StudyPremenopausal women require more levothyroxine than men or menopausal women, while all women require more levothyroxine than men, when using ideal weight-based formulae.
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