This study aims to investigate whether an earlier measurement of parathyroid hormone can reduce the number of patients who develop symptomatic hypoparathyroidism. This will provide evidence for a clear algorithm to address this issue after thyroid surgery. It will lead to an improvement in quality of life and a reduction in the distress experienced by affected patients. In addition, it is conceivable that patients' hospital stay could be shortened.

Information and Recruitment First, patients who meet the inclusion criteria and do not meet any exclusion criteria will be identified in our surgical-endocrine outpatient clinic. All concomitant medications, particularly any pre-existing vitamin D intake, will be documented. As part of the routine blood sampling, parathyroid hormone, calcium, and vitamin D are determined according to the guidelines in order to detect any pre-existing parathyroid disorders (exclusion criteria). After informing the patient about the indication, necessity, and risks of the planned procedure, an additional explanation is given regarding the possibility of participating in the study.

Both written and oral information about the study will be provided to the patient.

Study Timeline The planned recruitment period is 3 years, followed by a 6-month follow-up. Study Visits After patients are enrolled in the study, baseline characteristics are collected, as well as quality-of-life data using standardized questionnaires (SF-36, STAI-G) and the Health Patient Questionnaire (HPQ28), along with a modified version of the "Hypoparathyroidism Patient Experience Scale - Symptom (HPES)." This questionnaire is included in the appendices and has so far been validated for chronic hypoparathyroidism. Validation for the acute setting is currently being carried out through targeted surveys of our own patient cohort during routine perioperative visits.

Postoperative clinical visits take place on days 0, 1, and 2 in accordance with current clinical standards. In the intervention group, parathyroid hormone is measured directly postoperatively within the first 3 hours after surgery. In the control group, this measurement is performed on the first postoperative day (within 24 hours after surgery) during routine blood sampling. This measurement is also carried out at the same time point-on the first postoperative day-in the intervention group. In both groups, vitamin D and calcium supplementation is initiated if the parathyroid hormone level falls below the normal range. On postoperative days 1 and 2, both groups complete the HPES, HPQ28, and STAI-G questionnaires.

As part of the study, an additional clinical visit and laboratory testing is performed after discharge on days 8-10. This additional visit is normally conducted only in patients who showed abnormalities during their hospital stay, such as low calcium or parathyroid hormone levels.

Finally, after 6 months, patients are contacted again by telephone to assess the secondary endpoints, including a renewed evaluation of quality of life. The telephone follow-up also determines whether patients are still experiencing any symptoms at that time and whether continued vitamin D and calcium supplementation is necessary. For patients who did not develop postoperative hypoparathyroidism, the goal is to exclude the possibility that delayed symptoms have occurred (even though this is rare). Furthermore, it should be clarified whether early intervention to prevent symptomatic hypoparathyroidism leads to better long-term quality of life.

Patients in whom the condition persists long-term-i.e., after 6 months-will, as in current clinical routine, be referred to endocrinology for ongoing therapy and monitoring. There is already close collaboration with Prof. Dr. Stefanie Hahner and Prof. Dr. Carmina Fuß in endocrinology. This follow-up is no longer part of the study.

Together with the collaborating partners from endocrinology measurements of parathyroid hormone and calcium levels will be performed according to the UKW standard in the central laboratory and in the endocrinology laboratory. The standards and measurement methods in both laboratories are well established. No technical changes are expected in the coming years, as new devices for parathyroid hormone testing were recently acquired by the endocrinology department.

Randomization Randomization is carried out during or shortly after the surgical procedure by staff of the Surgical Study Center Würzburg. Randomization is performed using the platform "Randomizer.at." Registration takes place after approval by the ethics committee.

Inclusion Criteria:

• Age ≥ 18 years
• Thyroidectomy or bilateral thyroid resection of any indication
• Written informed consent obtained after detailed patient information and provision of an information sheet

Exclusion Criteria:

• Age ≤ 18 years
• Concomitant parathyroid disease (primary or secondary hyperparathyroidism, pre-existing hypoparathyroidism of any cause)
• Previous surgery in the neck region
• Pregnancy or breastfeeding
• Inability to attend study visits
• Participation in another study that interferes with the endpoints
• Missing or inadequate patient information

Individual participant data will not be shared due to data protection regulations and lack of participant consent for data sharing