Inclusion Criteria:

• 18 years of age or older.
• Cancer of the thyroid confirmed by a biopsy sample. Specific types of thyroid cancer included in this study are anaplastic, differentiated (papillary, follicular, or Hürthle cell), and medullary thyroid cancers.
• For patients with differentiated (papillary, follicular, or Hürthle cell) thyroid cancer, no more than 1 prior chemotherapy treatment is allowed.
• Measurable disease is required (at least one lesion at least 2 cm in length by conventional computed tomography (CT) techniques or at least 1 cm by spiral CT scan).
• Any prior chemotherapy or radiation therapy must be stopped at least 4 weeks before the first dose of study treatment. Prior radioiodine (I131) therapy must be stopped at least 3 (or 6) months before first dose of study treatment (depending on responsiveness to this therapy).
• Recovery from any toxic effects of prior chemotherapy, radiation therapy and surgery.
• Patients with reproductive potential must use a medically acceptable contraceptive method. Women of childbearing potential must have a negative pregnancy test at screening.

Exclusion Criteria:

• History of retinopathy.
• Serious ongoing medical or psychiatric disorder (as determined by the clinical investigator).
• External beam radiation therapy to >30% of the bone marrow at any time prior to study entry.
• Prior treatment with irofulven or capecitabine, or protracted infusion of 5-fluorouracil (5-FU) (infusion duration greater than or equal to 5 days) or other fluoropyrimidines.
• Therapeutic doses of coumarin derivatives (warfarin) 14 days prior to receiving the first dose of study treatment or during the study period.

Please note: There are additional criteria that must be met in order to be eligible for this study.