Eligible patients include those with previously untreated non-metastatic nasopharyngeal carcinoma (NPC) who will receive only one course of radical IMRT with or without adjunct chemotherapy for their NPC as curative treatment. Patients who have a history of any thyroid disorders, pituitary disorders, prior thyroid surgery or a drug history of thyroxine (T4) or triiodothyronine (T3) are excluded. Patients who have a history of radiation therapy to other areas are also excluded. Pretreatment investigations and workup include serum hematology, biochemistry, antibodies against Epstein-Barr virus (EBV) viral capsid antigen (VCA) and early antigen (EA), fabrication of customized head and neck thermoplastic cast for subsequent contrast-enhanced computed tomography (CT) scan of the head and neck region down to mid-thoracic region in IMRT treatment position with 3mm thickness, as well as T1-sequence, T2-sequence and gadolinium-enhanced magnetic resonance imaging (MRI) of the head and neck region by a 3-tesla scanner with the images co-registered with the planning head and neck CT images for detailed target and organ-at-risk (OAR) delineation and IMRT planning. A separate contrast-enhanced CT scan of the thorax and abdomen will also be performed to rule out distant metastasis.