BACKGROUND/AIM: The relationship between the hypothalamic-pituitary-thyroid (HPT) axis and the serotonergic (5-hydroxytryptamine, 5-HT) system is not clear. The aim of the study was to determine platelet biochemical markers (5-HT concentration and monoamine oxidase B, MAO-B, activity) in hypothyroid patients. ----- METHODS: The study included 25 medication-free female hypothyroid patients in postoperative follow-up after total thyroidectomy due to papillary thyroid carcinoma, who had not been treated with synthetic thyroxine (T(4)) for 4 weeks, and 44 age-matched euthyroid healthy women. The platelet 5-HT concentration, platelet MAO-B activity, total T(4) and thyroid-stimulating hormone (TSH) levels were determined using spectrofluorimetric methods, radioimmunoassay and fluoroimmunoassay, respectively. ----- RESULTS: Hypothyroid patients had significantly higher TSH, significantly lower T(4) levels and platelet 5-HT concentrations, and unchanged platelet MAO-B activity than healthy subjects. A positive correlation was found between the 5-HT concentration and platelet MAO-B activity, and between the platelet MAO-B activity and T(4) in control subjects. ----- CONCLUSIONS: Reduced platelet 5-HT concentrations in hypothyroid patients suggests a complex interaction between the 5-HT system and HPT axis activity, which could be related to the frequent occurrence of depressive symptoms in hypothyroid patients. The determination of platelet 5-HT concentrations should be considered a diagnostic tool for the evaluation of depressive symptoms in hypothyroid patients during the hormone withdrawal procedure.