Radiation-induced hypothyroidism in patients of oral squamous cell carcinoma: A retrospective analysis of 195 patients.

Background: Radiation-induced hypothyroidism (RIH) is common after neck irradiation, and biochemical evaluation of thyroid function is recommended periodically for early diagnosis and treatment. This study aimed to evaluate the predictors of RIH after completion of adjuvant radiotherapy (RT) for primary oral squamous cell carcinoma (OSCC).

Methods: This is a retrospective study involving 195 patients who received RT after surgery for OSCC between August 2011 and December 2016. Thyroid function tests were obtained every 6 months and patients were considered to be hypothyroid if thyroid-stimulating hormone level was >5 mIU/mL.

Results: The study cohort comprised 130 men with a median age of 52 years (range 21-77 years). About 107 (54.87%) patients developed hypothyroidism, with a median of 21 months (range 2-67 months) for the development of RIH. Women [41 (63.1%) versus 66 (50.8%), p=0.035], addition of chemotherapy [36 (63.2%) versus 71 (51.4%), p= 0.019], and higher cumulative dose to the thyroid gland (median dose 4690 cGy versus 2981 cGy, P < 0.001) resulted in higher incidence of RIH on univariate analysis. On multivariate Cox regression analysis, female sex (P = 0.042), bilateral irradiation (P = 0.046), and cumulative dose to the thyroid (P = 0.001) were factors associated with increased risk of developing RIH.

Conclusion: The addition of chemotherapy, high dose of radiation to the thyroid gland, bilateral irradiation, and female sex were at higher risk of developing RIH. However, more studies are required to identify the dose-volume constraints of the thyroid gland.