Your Dentist has informed you correctly. Though the procedure of Root Canal Treatment is safe... it is advisable to consult the physician under whom the thyroid problem of your wife is under treatment.

Yes anesthesia can be given.  Plain local anesthesia (without adrenaline) is usually preferred.  Root canal is a simple procedure that can be done in a single sitting as well. Don't worry. It is safe to go ahead with the procedure.

Thyroid patients are regularly treated in dental clinics on an OPD basis. The dentist would have suggested a physician's opinion probably because the TSH or T3/T4 values would have been outside the normal limit. If these reports are not normal, the general health of the patient is affected more severely than the dental health and needs to be taken care of first. Otherwise administration of local anaesthesia for dental extractions of root canal treatments is not different from that of a normal patient.