Background: The thyroid gland constantly regulates many body functions that help in the growth and metabolism, so any thyroid hormone disorder will affect these functions. However, treating cancer in the head and neck (HAN) region with radiotherapy may potentially harm the healthy cells of the thyroid gland. Aim: Hence, the current study aimed to evaluate radiation-induced thyroid disorder of head-and-neck cancer in children (≤18 years old, with normal TSH). Methods: The radiation-induced thyroid disorders were monitored by measuring the changes in TSH, FT3, FT4, Anti-TPO Ab and IGF-1 serum levels using ELISA kits in children. These biomarkers were measured at the baseline, after ten RT sessions (15–18 Grays) and after the radiotherapy (21–60 Grays). Three patient groups represent before, during and after the radiotherapy were compared with the control (healthy). Results: The results showed that TSH, IGF-1, and Anti-TPO Ab levels significantly decreased in the treated groups compared with the controls. However, FT4 increased significantly, and these changes weren't in a dose-dependent pattern. There was no significant difference in FT3 between the treated groups and the control. The TSH level in the patients after therapy (dose from 21 to 60 Grays) showed a significant negative correlation with the D mean and D50% of the thyroid, while the FT4 level showed a positive correlation. Conclusion: Radiotherapy of the head and neck region in children (≤ 18 years) may result in changing euthyroid to hyperthyroidism, which could be mainly attributable to the thyroid gland rather than the pituitary gland.