OBJECTIVE Because many patients with mental illness take more than one psychotropic drug, an understanding of the effects of every class of these drugs is very important to manage any thyroid abnormalities that can happen with these patients. METHODS A systematic review of all the published literature was made via Medline. Keywords used for the search were "thyroid side effects" in association with one of the following: "antipsychotics," "antidepressants," "lithium," "anticonvulsants," "benzodiazepines," "anticholinergics," "antihistaminergics," "cholinesterase inhibitors," "stimulants," "methadone," and "naltrexone." RESULTS Phenothiazines, which are antipsychotics, mainly alter iodine capture, complex and deactivate it, as well as decrease thyroid-stimulating hormone's (TSH's) response to thyroid-releasing hormone (TRH). Nonphenothiazines, typical antipsychotics, can induce the formation of thyroid autoantibodies and can elevate TSH levels. Atypical antipsychotics may decrease TRH-stimulated TSH. Tricyclic antidepressant drugs complex with iodine and thyroid peroxidase and deactivate them, induce deiodinase activity and interfere with the hypothalamo-pituitary-thyroid (HPT) axis by decreasing TSH response to TRH. The main effect with other antidepressant drugs is a decrease in circulating thyroid hormone levels. Lithium inhibits thyroid hormone release and increases TRH-stimulated TSH, inducing goiter, clinical and subclinical hypothyroidism, and hyperthyroidism. Carbamazepine mainly reversibly decreases serum thyroid hormone levels. Other psychotropic drugs such as valproic acid, benzodiazepines, opiates, anticholinergic and antihistaminergic drugs, and stimulants have minor interferences with thyroid functions. CONCLUSIONS Patients receiving lithium, phenothiazines, and tricyclic antidepressants TCA should be closely monitored for the development of thyroid function abnormalities. Only patients at risk for developing thyroid function abnormalities should be monitored when they receive typical and/or atypical antipsychotic drugs, nontricyclic antidepressant drugs, and carbamazepine. No specific recommendations are proposed as toward thyroid function monitoring for patients receiving any other psychopharmacologic drug.