SLC6A4 and fibromyalgia: In the present study, both the expression and functionality of SERT were determined in platelets collected from patients with FM, with the following aims: first, to perform a comparison with the pharmacological profile of platelet SERT in healthy volunteers, and second, to examine putative correlations of SERT characteristics with the severity of symptoms such as tiredness, Fibromyalgia Impact Questionnaire (FIQ) score or tender point index (TPi).