Relationship of Isthmus Thickness With Difficult Laryngoscopy and Difficult Intubation in Patients Receiving Hypothyroidism Treatment

Under general anesthesia, the rate of failed intubation ranges from 1.5% to 13%, raising concerns among anesthesiologists whose primary goal is successful airway management. In this context, various physical and ultrasonographic measurement techniques have been developed to predict difficult intubation. Ultrasonography is effectively used for estimating tracheal tube size, device placement, diagnosing upper airway pathologies, and guiding percutaneous tracheostomy. In thyroid pathologies, intubation difficulty may increase; however, the impact of goiter remains debatable, as some conditions causing hypothyroidism are reported to lead to thyroid gland atrophy rather than hypertrophy. The study's hypothesis is that an atrophic or fibrotic thyroid isthmus may be associated with difficult laryngoscopy and intubation. Evaluation will be performed using the Cormack-Lehane score (Grade III-IV) and the Intubation Difficulty Scale (IDS \>5). The aim is to determine the relationship between thyroid isthmus thickness and difficult laryngoscopy and intubation during elective intubation in patients receiving hypothyroidism treatment. Preoperatively, patients' demographic and clinical data (age, gender, height, weight, BMI, comorbidities, ASA score, thyroid medication dose, treatment duration, and type of thyroid disease) will be recorded. In the premedication room, after administering 0.01 mg/kg IV midazolam, the distance between the thyroid isthmus and the skin will be measured using a linear ultrasound probe (3-13 Hz) in the supine position with neck hyperextension at the level of the 2nd-3rd tracheal rings; the average of three measurements will be recorded. In the operating room, under noninvasive monitoring and following mask pre-oxygenation, anesthesia induction will be performed using IV 2 mg/kg propofol, 1 µg/kg fentanyl, 1 mg/kg lidocaine, and 0.6 mg/kg rocuronium. Once the TOF reaches zero, an experienced anesthesiologist will intubate using a size 3 Macintosh blade f