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Background: Many features of fibromyalgia and hypothyroidism are virtuallythe same, and thyroid hormone treatment trials have reduced or eliminated fibromyalgia symptoms. Thesefindings led the authors to test the hypothesis that fibromyalgia patients are hypometabolic comparedto matched controls. Material/Methods: Resting metabolic rate (RMR) was measured by indirect calorimetryand 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-freeweight (FFW) (Sterling-Passmore: SP) and by sex, age, height, and weight (Harris-Benedict: HB). Results:Patients had a lower mRMR (4,306.31+/-1077.66 kJ vs 5,411.59+/-695.95 kJ, p=0.0028) and lower percentagesof pRMRs (SP: -28.42+/-15.82% vs -6.83+/-12.55%, p3[/sub] (FT[sub]3[/sub]) accounted for 30% of the variance in pressure-pain threshold.Conclusions: Patients had lower mRMR and percentages of pRMRs. The lower RMRs were not due to calorierestriction or low FFW. Patients' normal FFW argues against low physical activity as the mechanism. TSH,FT[sub]4[/sub], and FT[sub]3[/sub] levels did not correlate with RMRs in either group. This does not rule out inadequatethyroid hormone regulation because studies show these laboratory values do not reliably predict RMR.