Thesis (M.Med. (Radiation Oncology)) -- University of Limpopo, 2022 / Background Hypothyroidism in head and neck cancer patients after radiotherapy is known to occur, yet thyroid function tests are not routinely monitored in all patients post radiation therapy. Routine post radiation therapy thyroid function testing is currently not part of the follow-up protocol in these patients at Pietersburg Hospital. The aim of this study is to evaluate post radiation therapy hypothyroidism among head and neck cancer patients treated with radiotherapy at Pietersburg Hospital Methods A prospective (cohort) observational study was carried out among head and neck cancer patients receiving radiotherapy at the radiation oncology department in Pietersburg Hospital. Sample size of n=37 was calculated using Statistica V13.0. Thyroid function tests were performed at the start of radiation therapy and repeated on the first day of follow up, 6 weeks after completing radiation therapy. During follow up, participants were also interviewed for the presence of symptoms of hypothyroidism such as dry skin, dry hair, fatigue, cold intolerance, or weight gain. Data analysis was done with STATA version 16. Descriptive statistics were used to characterise variables, and summarised in tables, graphs and charts. Changes in thyroid function tests and other variables were analysed. A p-value of 0.05 was deemed statistically significant. Results Thirty-seven patients were enrolled in the study, 26 males and 11 females. The mean age of the patients was 53.1 ±12.3 standard deviation [SD]) with a range of 40.8 to 65.4 years. The most common diagnoses were cancer of the larynx and hypopharynx, forming 29.7% and oral cavity cancer, 29.7%. Only three patients (8%) had an early stage cancer (Stages 1 and 2), 11 patients (29.7%) moderately advanced cancer (Stage 3) while the majority (62%; n =23) had locally advanced cancer (Stage 4). Majority of the patients received 70Gy in 35 daily fractions, five fractions per week via 3-D conformal radiotherapy. Only 29 patients who had complete pre- and post radiotherapy thyroid function tests were included in the final analysis. Of these, none had clinical hypothyroidism at 3 months. Two patients (6.8%) had sub-clinical hypothyroidism, with post radiation therapy TSH values greater than 3.5mIU/ml. The mean post radiation therapy TSH values increased by 8.3% and the mean fT4 values decreased by 2.05% compared to the pre-radiation therapy values. Both changes were not statistically significant (p=0.99 and p=0.82 respectively). There was no statistically significant correlation between changes in TSH and fT4 versus age (p=0.88 and p=0.92 respectively), sex (p=0.55 and p=0.15 respectively), cancer stage (p=0.21 and p=0.78 respectively), and cancer site (p=0.17 and p=0.74 respectively). The most common post radiotherapy symptom was fatigue (62%) followed by cold intolerance (54%), weight gain (43%) and dry skin or dry hair (43% each). Conclusion The results of the study suggest that sub-clinical hypothyroidism is detectable early post radiation therapy presenting as clinical symptoms.