Observing the need for general anesthesia with monitorization in patients with normal thyroid function and diagnosed with thyroid function disorder

Introduction and objective: Hypothyroidism is a common endocrine disorder resulting from insufficient hormone production by the thyroid gland. Structural and functional changes in the brain have been demonstrated in hypothyroid patients using multimodal imaging techniques. Studies have supported a reciprocal regulation between thyroid hormones and the GABAergic system. Propofol, the most commonly used intravenous induction agent, acts through GABA receptors. The primary objective of our study is to determine whether there is a distribution in the required propofol dose under total intravenous anesthesia, guided by variable bispectral index monitoring, in relation to thyroid hormone levels. Our secondary objective is to investigate whether there is a difference in the amount of the GABA metabolite in the anterior and posterior cortex regions on magnetic resonance spectroscopy imaging between patients with normal and abnormal thyroid function. Materials and Methods: A total of 80 patients with ASA I-II status were included in our study: Group 1, the Control group (n=40), consisted of patients without thyroid dysfunction, and Group 2, the Hypothyroid group (n=40), included patients diagnosed with hypothyroidism who were taking exogenous thyroid hormones and had thyroid function tests within normal ranges. Preoperatively, patients were assessed using the Montreal Cognitive Assessment Scale and the Beck Anxiety Inventory. Brain MRI scans were performed on all patients one day before surgery. Magnetic resonance spectroscopy imaging was chosen due to its non-invasive nature and its effectiveness in evaluating brain metabolite changes in patients with thyroid dysfunction. In examining brain metabolites, NAA, Cr, and Glx peaks were identified, which were interpreted as indicators of changing GABA concentrations. The patients were operated on using total intravenous anesthesia. Induction was made with 2 mg/kg propofol, 0.5 mg/kg fentanyl, and 0.5 mg/kg rocuronium. Maintenance bispectral index monitoring is performed with propofol infusion at a dose of 1-8 mg/kg/min and remifentanil infusion at a dose of 0.5-2 mcg/kg/min, with bispectral index monitoring values between 40-60. Results: In the patient group diagnosed with hypothyroidism, mean age, female gender and ASA II rates were found to be statistically significantly higher compared to the control group (p values; p<0.001, p<0.001, p=0.002, respectively). In the patients included in the study, TSH level was found to be statistically significantly higher compared to the control group, and T3 level was found to be statistically significantly lower compared to the control group (p values; p<0.001, p=0.020, respectively). In the patient group diagnosed with hypothyroidism, GABA metabolite levels in the mid anterior cingulate cortex (mACC) and mid posterior cingulate cortex (mPCC) were found to be statistically significantly higher compared to the control group (p<0.001, p<0.001). No statistically significant correlation was found between GABA metabolites in the mACC and mPCC and thyroid hormone levels in the patient and control groups (p>0.005). In the control group patients, a negative and good statistically significant correlation was detected between propofol levels and T4 (p<0.001). In the control group patients, a positive and low-moderate statistically significant correlation was detected between GABA metabolite levels in the mid posterior cingulate cortex and propofol levels (p=0.038). Conclusion: Our study is the first in the literature that provides a comprehensive evaluation of both the determination of GABA receptors in patients diagnosed with hypothyroidism and the comparison of intraoperative anesthetic drug usage. In our study, patients diagnosed with hypothyroidism were found to have significantly higher serum TSH hormone levels and GABA metabolite levels in the anterior cingulate cortex and posterior cingulate cortex of the brain. However, it was observed that the amount of propofol and remifentanil used did not change significantly. In the healthy group, a strong correlation was observed between thyroid hormone levels and the amounts of propofol and remifentanil used. In contrast, this correlation was not observed in the hypothyroid group using exogenous thyroid hormones. More studies are needed to fully understand the effects of hypothyroidism due to the complexity of its neurological effects and interindividual differences. Keywords: Anesthesia, GABA metabolite, Hypothyroidism, Propofol, Remifentanil, Magnetic Resonance Spectroscopy