The authors sought to determine the incidence of lithium-induced hypothyroidism in a population-based cohort of older adults beginning lithium therapy, and thereby to inform clinical guidelines on the frequency of monitoring necessary in this group.
The authors conducted a population-based observational cohort study using four administrative databases that contained information on over 1.3 million older adults in Ontario who receive universal healthcare coverage in terms of physician services, drugs, and hospitalizations. Over an 18-month period, they studied adults aged 65 years and over who were newly prescribed lithium or valproate, monitoring subjects for initiation of T4 therapy (as a proxy for hypothyroidism) while they continued their lithium use. The authors identified 1,705 new users of lithium and 2,406 new users of valproate with similar baseline characteristics.
Lithium users were significantly more likely to be treated with T4 than were valproate users. The rate of T4 treatment per "100 person-years" was 5.65 in the lithium group and 2.70 in the valproate group. T4 treatment was initiated in almost 6% of lithium-treated patients, suggesting the possibility that hypothyroidism developed twice as frequently among these patients as would be expected among a mixed-age population.
Increased vigilance and continued monitoring of thyroid functioning for at least two years is necessary in older adults beginning lithium therapy.