Pam --
My mother began taking synthetic thyroid pills after her thyroid was removed in the early 1960s (yes, more than 40 years ago), and my husband has taken them for a "sluggish thyroid" for about five years. Your doctor will periodically do tests to fine-tune the dosage -- for example, my husband takes one pill every other day -- but neither has had any problems with the medication.
All the best,
Leslie