Treated hypothyroidism, cognitive function, and depressed mood in old age: the Rancho Bernardo Study.

OBJECTIVE: Overt hypothyroidism is associated with cognitive impairment, which can be reversed if treated early and appropriately. We compared cognitive function (CF) of euthyroid older adults with those who had long-term treated hypothyroidism.
METHODS: Between 1999 and 2003, the CF of 885 euthyroid and 149 hypothyroid-treated older adults (primary hypothyroidism after surgery or autoimmune thyroid disease) was assessed using three standardized CF tests: the modified mini-mental state examination, Trails B, and verbal fluency. Depressed mood was assessed using the Beck Depression Inventory (BDI). Only participants with thyroid stimulating hormone (TSH) in the normal range were included.
RESULTS: The treated hypothyroid group had been treated with l-thyroxine for an average of 20 years. Those with treated hypothyroidism were older than the euthyroid group (76.1+/-9.6 vs 73.6+/-10.2 years, P=0.005) and were much more often women (81.6 vs 54.8%, P<0.001). TSH levels were similar between groups (median interquartile range=1.57 (1.19) vs 1.54 (1.59) mIU/l, P=0.81). Compared to euthyroid, the treated hypothyroidism group had more frequent antidepressant medication use (19.5 vs 8.5%, P<0.001) but similar BDI scores. Performance on the three CF tests did not differ by thyroid hormone treatment. Results were not changed after adjustment for age, sex, antidepressant medication use, exercise, and total cholesterol.
CONCLUSION: Long-term treated hypothyroidism is not associated with impaired CF or depressed mood in old age. The lack of association with CF is reassuring with regard to long-term use of thyroid hormone therapy.