Levothyroxine dose following thyroidectomy is affected by more than just body weight

To determine the factors that affect levothyroxine (LT4) requirements following thyroidectomy.

Retrospective study.

This study evaluated 246 participants who had undergone total thyroidectomy and were on a stable dose of LT4. Actual weight-based (AWB) and ideal body weight-based (IBWB) LT4 dose requirements were analyzed, and other confounders including adherence, concurrent medications, comorbidities, female menopausal status, and hormone replacement therapy were examined.

A total of 205 women and 41 men were evaluated, with 48 (20%) benign and 198 (80%) malignant pathology findings. The mean AWB LT4 doses for men and premenopausal women were similar among members of the benign groups and similar among members of the malignant groups. There was a trend for lower dose LT4 in postmenopausal women off hormonal therapy (PM/NH) and on hormonal therapy (PM/H) in the benign group (1.4 and 1.6 μg/kg vs. 1.8 μg/kg in the men and premenopausal women) and a trend for lower dose LT4 in the PM/H women in the malignant group (1.9 μg/kg vs. 2.1 and 2.2 μg/kg in the men and premenopausal women), but they were not significant. However, PM/NH women required significantly less LT4 (1.7 μg/kg) than both the men (2.2 μg/kg) and premenopausal women (2.1 μg/kg) in the malignant group (P=.0006). The IBWB LT4 dosage was not statistically different between groups.

LT4 dosage following thyroidectomy, calculated using actual body weight, can range from 1.4 to 2.2 μg/kg and is dependent on diagnosis (benign vs. malignant), goal TSH, sex, and menopausal status.