One nonrandomized control study (VanPelt RE, et al, 2000) of neutral research design quality reported group mean differences in adjusted RMR (adjusted for fat-free mass and fat mass) were not associated with differences in plasma concentrations of total T₃ or total T₄ (P>0.05) in 15 post-menopausal sedentary females (ages 50-72) compared to 12 pre-menopausal sedentary females, ages 21-25. In 121 post-menopausal (49-58 years), nonobese and obese women (BMI 25-42 kg/m2) (Svendsen, 1993), RMR correlation with total triiodothyronine (T₃) was statistically significant (r=0.17; P<0.057); when assessed by stepwise multiple regression (i.e., independent predictors). Total T₃ separately, explained an additional 5% of the variation and was statistically significant (P<0.057) and independently associated with RMR. Overall, this study suggests that in healthy premenopausal and postmenopausal, normal and overweight women, the inter-individual difference is mainly due to variation in lean tissue mass, and to some extent variation in fat tissue mass, T3 and androstenedione.

Note: Not graded due to close similarity to overall conclusion statements.

In a meta-analysis (Astrup, 1996) of neutral research quality design, group mean RMR was 8% lower in 28 weight stable, post-obese women than in 28 control subjects (p<0.02) (group mean % body fat of 27.2 and 26, respectively and matched for age, fat mass, and fat-free mass (FFM); and, age range of 35-41 years) and the group mean difference remained unchanged after fat-free mass and fat mass were adjusted for (131 kcal/day, p<0.015). A statistically-significant (P<0.01) lower concentration of free triiodothyronine occurred in post-obese women compared to matched controls; even though mean free triiodothyronine levels were within normal limits in both groups. There were no significant correlations between plasma free triiodothyronine concentration and RMR when adjusted for differences in FFM and fat mass (r=0.17, p=0.13; i.e., explains 3% of variance).

In a randomized controlled study (Kong, 2002) of neutral research design quality, 16 women (mean age of 53± 3 y; BMI 25.5± 3.2 kg/m²) with subclinical hypothyroidism [i.e., normal serum free thyroxine (T₄) level and a serum thyroid-stimulating hormone level (TSH level) of 5 to 10 microUnits/milliliter] received a 6-month treatment of 50-100 micrograms thyroxine to return serum levels to normal and treatment did not significantly change lean body mass percent (0.1%; 95% confidence interval -1.6-1.7%) nor RMR (-0.2 kcal/kg/24 hr; 95% confidence interval -1.3 to 1.0 kcal/kg/24 hr) and was not statistically different compared to the 17 women who received a placebo of ascorbic acid (P>0.05). There were no significant correlations between TSH levels measured at 6 months and changes in RMR (r=-0.3, P=0.2).

Note:  Not graded due to close similarity to overall conclusion statements.

In individuals who have lost weight (i.e., post-obese) or are known to have subclinical levels of thyroid hormones, RMR adjusted for lean body mass does not correlate with thyroid hormone status.