Although the interactions between lithium treatment and thyroid function have long been recognised, their clinical relevance is still controversial. This paper sets out a review of the literature to date, considering that lithium still represents the gold standard among prophylactic treatments of manic-depression several decades after its introduction.
PubMed database was used to search for English-language articles relating to lithium treatment and thyroid function. As the amount of relevant papers totalled several hundreds, this review refers to previous reviews, especially with regard to older literature. Moreover, the authors particularly refer to a series of studies of thyroid function performed in a cohort of patients at different stages of lithium treatment, who were followed up by their group from 1989 onwards.
The main findings from this review included: a) lithium definitely affects thyroid function as repeatedly shown by studies on cell cultures, experimental animals, volunteers, and patients; b) inhibition of thyroid hormone release is the critical mechanism in the development of hypothyroidism, goitre, and, perhaps, changes in the texture of the gland which are detected by ultrasonic scanning; c) compensatory mechanisms operate and prevent the development of hypothyroidism in the majority of patients; d) when additional risk factors are present, either environmental (such as iodine deficiency) or intrinsic (immunogenetic background), compensatory potential may be reduced and clinically relevant consequences may derive; e) hypothyroidism may develop in particular during the first years of lithium treatment, in middle-aged women, and in the presence of thyroid autoimmunity; f) thyroid autoimmunity is found in excess among patients suffering from affective disorders, irrespective of lithium exposure; g) in patients who have been on lithium for several years, the outcome of hypothyroidism, goitre, and thyroid autoimmunity do not much differ from those observed in the general population; h) hyperthyroidism and thyroid cancer are observed rarely during lithium treatment.
Thyroid function tests (TSH, free thyroid hormones, specific antibodies, and ultrasonic scanning) should be performed prior to starting lithium prophylaxis. A similar panel should be repeated at one year. Thereafter, annual measurements of TSH may be sufficient to prevent overt hypothyroidism. In the presence of raised TSH or thyroid autoimmunity, shorter intervals between assessments are advisable (4–6 months). Measurement of antibodies and ultrasonic scanning may be repeated at 2-to-3-year intervals. The patient must be referred to the endocrinologist if TSH concentrations are repeatedly abnormal, and/or goitre or nodules are detected. Thyroid function abnormalities should not constitute an outright contraindication to lithium treatment, and lithium should not be stopped if a patient develops thyroid abnormalities. Decisions should be made taking into account the evidence that lithium treatment is perhaps the only efficient means of reducing the excessive mortality which is otherwise associated with affective disorders.