Many features of fibromyalgia and hypothyroidism are virtually the same, and thyroid hormone treatment trials have reduced or eliminated fibromyalgia symptoms. These findings led the authors to test the hypothesis that fibromyalgia patients are hypometabolic compared to matched controls.
Resting metabolic rate (RMR) was measured by indirect calorimetry and body composition by bioelectrical impedance for 15 fibromyalgia patients and 15 healthy matched controls. Measured resting metabolic rate (mRMR) was compared to percentages of predicted RMR (pRMR) by fat-free weight (FFW) (Sterling-Passmore: SP) and by sex, age, height, and weight (Harris-Benedict: HB).
Patients had a lower mRMR (4,306.31+/-1077.66 kJ vs 5,411.59+/-695.95 kJ, p=0.0028) and lower percentages of pRMRs (SP: -28.42+/-15.82% vs -6.83+/-12.55%, p<0.0001. HB: -29.20+/-17.43% vs -9.13+/-9.51%, p=0.0008). Whereas FFW, age, weight, and body mass index (BMI) best accounted for variability in controls' RMRs, age and fat weight (FW) did for patients. In the patient group, TSH level accounted for 28% of the variance in pain distribution, and free T3 (FT3) accounted for 30% of the variance in pressure-pain threshold.
Patients had lower mRMR and percentages of pRMRs. The lower RMRs were not due to calorie restriction or low FFW. Patients' normal FFW argues against low physical activity as the mechanism. TSH, FT4, and FT3 levels did not correlate with RMRs in either group. This does not rule out inadequate thyroid hormone regulation because studies show these laboratory values do not reliably predict RMR.