Brief Answer:
Thyroid and Lithium

Detailed Answer:
There are many factors that determine whether or not thyroid function will return to normal after cessation of lithium carbonate.

For example, the chances of goiter (thyroid swelling) and hypothyroidism (under-active thyroid) occurring after starting lithium are highest in the first couple years.

So if you are assuming 6 months therapy and then stopping it, the chances of significant thyroid damage occurring later are small, if it did not occur during the time lithium was given.

However lithium has complex effects on the thyroid gland, some of which are not fully understood.
Another factor is thyroid autoimmunity ie the immune process that causes thyroid glands to become inactive, has been shown to be associated with lithium therapy. So if these antibodies are evident after lithium is started it id possible they may persist later and cause hypothyroidism later even after lithium is stopped.

Furthermore, lithium induced thyroid damage may occur more commonly in those with pre-existing risk factors for thyroid disease such as thyroid auto-immnunity (as described above).

So there is no long term systematic scientific published study, to the best of my knowledge in medical literature, that had sought to answer your specific question : how long after cessation of lithium therapy does thyroid function recover if at all, and whether there are irreversible changes in the thyroid.

The best approach is to monitor thyroid function periodically and see an endocrinologist regularly to have a thorough thyroid examination

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