Effects of levothyroxine replacement or suppressive therapy on energy expenditure and body composition.

Effects of levothyroxine replacement or suppressive therapy on energy expenditure and body composition.

Thyroid. 2015 Dec 23;

Authors: Samuels M, Kolobova I, Smeraglio A, Peters D, Purnell J, Schuff KG

Abstract
BACKGROUND: TSH-suppressive doses of levothyroxine (L-T4) have adverse effects on bone and cardiac function, but it is unclear whether metabolic function is also affected. The objective of this study was to determine whether women receiving TSH-suppressive L-T4 doses have alterations in energy expenditure or body composition.
METHODS: This study was a cross-sectional comparison among three groups of women: 26 women receiving chronic TSH-suppressive L-T4 doses, 80 women receiving chronic replacement L-T4 doses, and 16 untreated euthyroid control women. Subjects underwent measurements of resting energy expenditure (REE), substrate oxidation, and thermic effect of food by indirect calorimetry; physical activity energy expenditure by accelerometer; caloric intake by 24-hour diet recall; and body composition by dual x-ray absorptiometry.
RESULTS: REE per kilogram lean body mass in the L-T4 euthyroid women was 6% lower than the L-T4 suppressed group and 4% lower than the healthy control group (P=0.04). Free T3 levels were directly correlated with REE, and were 10% lower in the L-T4 euthyroid women compared to the other two groups (P=0.007). The groups of subjects did not differ in other measures of energy expenditure, caloric intake, or body composition.
CONCLUSIONS: L-T4 suppression therapy does not adversely affect energy expenditure or body composition in women. However, L-T4 replacement therapy is associated with a lower REE despite TSH levels within the reference range. This may be due to lower fT3 levels, suggesting relative tissue hypothyroidism may contribute to impaired energy expenditure in L-T4 therapy.

PMID: 26700485 [PubMed - as supplied by publisher]

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