Antithyroid Drugs

• Methimazole, or Tapazole, and Propylthiouracil, usually called PTU, are antithyroid drugs used to treat hyperthyroidism. One of the possible side effects of these medications is hives.
  
  

Dermatitis Herpetiformis

• Dermatitis herpetiformis is a condition that can be associated with autoimmune thyroid disease. Hives on your back and lower extremities are one of the symptoms of this condition.
  
  

Chronic Urticaria

• Chronic urticaria is another term for chronic hives. It is a condition that increases your risk of thyroid imbalance, according to Dr. Ridha Arem's "The Thyroid Solution: A Revolutionary Mind-Body Program for Regaining Your Emotional and Physical Health."
  
  

RAI

• RAI, or radioactive iodine treatment, is a common treatment for hyperthyroidism and sometimes used as a tool to determine how well your thyroid is functioning. Along with tenderness to your thyroid, nausea and bladder irritation, hives are a possible, if rare, side effect.
  
  

Allergic Reaction

• If you develop hives as a reaction to thyroid medication, consult your doctor, as he may want to discontinue your medication program and try a different treatment.