Effect of hyper- and hypothyroidism on platelet monoamine oxidase activity and serotonin metabolism.

We evaluated platelet monoamine oxidase (MAO) activity in 13 hyperthyroid and 9 hypothyroid patients. The platelet MAO activity of these patients did not differ from that of age- and sex-matched euthyroid control subjects. Nineteen hyperthyroid and twelve hypothyroid subjects had serum serotonin concentrations similar to those of age- and sex-matched euthyroid control subjects. We also determined the urinary excretion of 5-hydroxy-indoleacetic acid (5-HlAA), serotonin, tryptamine, and tyramine in five hyperthyroid and four hypothyroid subjects. Two hyperthyroid patients had increased tyramine excretion; one of these patients also had increased tryptamine excretion. Three hypothyroid patients and one hyperthyroid patients had slightly decreased serotonin excretion. Both the hyper- and the hypothyroid subjects had normal urinary 5-HIAA excretion. There was no relationship between platelet MAO activity and monoamine excretion in the patients with thyroid dysfunction. We conclude that there is no systematic change in platelet MAO activity or serum serotonin concentration in patients with hyper- or hypothyroidism.