Hypothyroidism frequently occurs in hyperthyroid individuals. It may happen even in the absence of treatment or when treatment is used, such as anti-thyroid medications (methimazole or propylthiouracil) putting the gland at rest, or radioiodine therapy, or surgery. The two latter, destroying or removing partially thyroid tissue, increase the incidence of hypothyroidism and shorten the interval before it appears. With anti-thyroid medications, it may occur as late as twenty to thirty years.
In the elderly person, the first line of treatment is often radioiodine therapy, on the contrary of what is done in young patient. A retrospective study, from April 1999 to February 2004, carried out at the Nuclear Medical Center Jean Perrin at Clermont-Ferrand, on 270 hyperthyroid patients (including 150 patients over 65 years old), shows, after 12 months of follow-up, 56 % of return to euthyroidism (normal thyroid function) in elderly patients, while 73.6 % cases of the younger patients were hypothyroid. A lower incidence of Graves' hyperthyroidism among elderly people explained this result.