To Compare The Effects Of Two Doses Of Vandetanib In Patients With Advanced Medullary Thyroid Cancer

Inclusion Criteria: * Written consent from Female or male patients aged 18 years and over. Previously confirmed histological diagnosis of unresectable, locally advanced or metastatic, hereditary or sporadic MTC Objective disease progression within the previous 14 months prior to enrolment, and/or * Have one or more symptoms that the Investigator believes to be related to the patient's MTC. * World Health Organisation (WHO) or Eastern Cooperative Oncology Group (ECOG) Performance status 0-2. * Has measurable disease (at least one lesion, not irradiated within 12 weeks of study randomisation, with longest diameter more or equal 10mm (lymph nodes minimum more or equal 15 mm) with CT or MRI). * Lesions must be amenable to accurate and repeat measurement. Exclusion Criteria: * Prior treatment (major surgery, radiation therapy, chemotherapy, or other investigational drugs) received within 28 days before randomization. * Abnormal liver function tests (bilirubin more than 1.5xULRR, and ALT, AST, or ALP more than 2.5xULRR or 5.0xULRR if related to liver metastases). * Significant cardiac conditions or events such as reduced cardiac functions, symptomatic cardiac arrhythmia requiring treatment, congenital long QT syndrome, history of drug-induced QT prolongation, or QTcF correction unmeasurable or more than 450 ms. * Abnormal electrolytes such as potassium, magnesium and calcium, or abnormal organ functions such as decreased creatinine clearance. * For women only - currently pregnant or breast feeding.