Serotonin levels, pain threshold, and fibromyalgia symptoms in the general population

To investigate the association of serum serotonin with fibromyalgia (FM), and to study the relationship of serotonin to clinical variables associated with FM.

Serum samples (n = 292) were obtained on subjects without pain, with regional pain, and with widespread pain during a population survey. The tender point examination was made according to American College of Rheumatology examination criteria by an examiner blind to the subjects' complaints. Serotonin was determined by high performance liquid chromatography coupled to an electrochemical detector.

No associations between clinical variables and serotonin levels were found in the group as a whole. Subjects with FM had lower serotonin levels unadjusted (p = 0.019) and adjusted for age and sex (p = 0.059) than those without FM. Within the FM group, associations between serotonin and tender point count (r = 0.563) and depression (r = 0.549) were noted, but the direction of association was opposite to previous reports and expectations.

Serum serotonin levels are significantly lower in persons with FM compared to those without FM, but the range of values is wide. Difficult to explain correlations with reversed directions are noted for tender point count, dolorimetry, depression, and anxiety among those with FM. Serotonin is not correlated with any clinical variables in the general population, and separate pain groups cannot be distinguished.