Incidence and risk factors of hypothyroidism in psychiatric outpatients with long-term lithium treatment: a 10-year retrospective hospital-based study.

Background: The study aimed to assess the incidence of lithium-induced hypothyroidism and its associated factors in psychiatric outpatients on long-term lithium therapy.

Methods: A retrospective study at Songklanagarind Hospital (January 1, 2013, to September 30, 2022) focused on clinical (overt) and subclinical forms of lithium-induced hypothyroidism. Subclinical hypothyroidism, characterised by elevated thyroid stimulating hormone (TSH) but normal thyroxine (FT4) levels, and clinical hypothyroidism, by low FT4 and high TSH, were analysed. Patients with less than 3 months of lithium therapy or prior thyroid conditions were excluded. We used survival analysis and logistic regression to investigate incidence and factors.

Results: Among 461 on lithium maintenance therapy, 279 outpatients met the inclusion criteria; however, only 166 (59%) were monitored for thyroid functions during the study period and included in the final analyses, mostly female (53.6%) with bipolar disorder (72.9%), median age 42.0 years. The median duration of maintenance therapy was 3.7 years (IQR = 1.0-9.3). Over a decade, 30 patients developed lithium-induced hypothyroidism, primarily subclinical (28 cases), at an incidence rate of 0.0212 cases per patient-year. Hypothyroidism manifested after 14.4 months of lithium therapy (IQR = 5.2-53.2), with a prominent decline of survival curve within the first year. Younger age (adjusted OR = 0.95, P = 0.001), presence of physical comorbidities (adjusted OR = 2.69, P = 0.039), and higher lithium levels (P = 0.003) were associated with hypothyroidism.

Conclusions: Despite low incidence, regular thyroid monitoring is advised for patients on lithium. The study identifies risk factors for lithium-induced hypothyroidism, aiding in patient risk assessment and monitoring protocols.