Yet another paper questioning the effect of levothyroxine monotherapy on free T3 levels and, therefore, on metabolism. With that wonderful term "relative tissue hypothyroidism".
Thyroid. 2015 Dec 23. [Epub ahead of print]
Effects of levothyroxine replacement or suppressive therapy on energy expenditure and body composition.
Samuels M1, Kolobova I2, Smeraglio A3, Peters D4, Purnell J5, Schuff KG6.
1Oregon Health Sciences University , 3181 SW SamJackson Park Road , Portland, Oregon, United States , 97221 , (503) 494-5242 , (503) 494-0165 ; firstname.lastname@example.org.
2Oregon Health & Science University, Endocrinology, Diabetes and Metabolism, Portland, Oregon, United States , 206-859-8317 ; email@example.com.
3Stanford University School of Medicine, Stanford, California, United States , 650-723-4000 ; firstname.lastname@example.org.
4Oregon Health & Science University, Division of Biostatistics, Portland, Oregon, United States , 503-494-9000 ; email@example.com.
5Oregon Health & Science University, Endocrinology, Diabetes and Metabolism, Portland, Oregon, United States , 503-494-9000 ; firstname.lastname@example.org.
6Oregon Health Sciences University, Medicine/Endocrinology , 3181 SW Sam Jackson Park Road , L-607 , Portland, Oregon, United States , 97239-3098 , (503) 494-1685 , (503) 494-6990 ; email@example.com.
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.
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.
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.
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]