Association between thyroid hormone levels and monoaminergic neurotransmission during surgery

BACKGROUND: Human studies assessing thyroid hormone metabolism in relation to brain monoaminergic activity in vivo are scarce. The few studies that do exist suggest significant associations between thyroid function and monoaminergic activity, but the cause-and-effect relationships are far from elucidated. METHODS: We simultaneously collected cerebrospinal fluid (CSF) and serum samples from 35 patients undergoing orthopaedic surgery before, 3h after and the morning after interventions and performed analyses for thyroid hormones and monoamine metabolites. RESULTS: At baseline, the CSF 3-methoxy-4-hydroxyphenylglycol concentrations were significantly correlated to the serum T(3)/T(4) ratio (rho=0.41, p=0.017). During surgery, serum thyroid hormones and the T(3)/T(4) ratio decreased (p<0.0001), while the CSF T(3)/T(4) ratio increased (p=0.0009). There were no correlations between serum and CSF levels of T(3) and T(4) at any of the samplings. Strong correlations were noted between baseline CSF thyroid hormone concentrations and subsequent increases in CSF 5-hydroxyindoleacetic acid (5-HIAA), and homovanillinic acid (HVA), but not vice versa. CONCLUSIONS: Thyroid hormone levels in serum and CSF during stress seem to be distinctly regulated. Baseline thyroid hormone activity may facilitate changes in brain monoaminergic neurotransmission in response to stress.