Evaluation of Efficacy, Safety of Vandetanib in Patients With Differentiated Thyroid Cancer

Primary Objective: To determine the efficacy (as assessed by progression-free survival \[PFS\]) of vandetanib when compared to placebo in participants with differentiated thyroid cancer that is either locally advanced or metastatic who are refractory or unsuitable for radioiodine therapy. Secondary Objectives: * To determine the efficacy of vandetanib when compared to placebo in this participant population as assessed by efficacy variables including duration of response (DOR), objective response rate (ORR), change in tumour size (TS) and overall survival (OS). * To evaluate the pharmacokinetics (PK) of vandetanib in this participant population and potentially investigate any influence of participant demography and pathophysiology on vandetanib PK. * To demonstrate an improvement in time to worsening of pain (TWP) in participants treated with vandetanib when compared to placebo in this participant population. * To evaluate the safety and tolerability of vandetanib treatment in this participant population.