Thyroid disease and mental disorders: cause and effect or only comorbidity?

To discuss the effects of thyroid dysfunction and thyroid autoimmunity on mental symptoms and disorders in patients with thyroid disease with reference to recent epidemiological, clinical, and genetic findings.

During brain development, iodine deficiency, maternal thyroid dysfunction, and neonatal thyroid malformations together with genetic factors contribute to neurological deficit. Most adults with thyroid dysfunction will develop mental symptoms. In hyperthyroidism, adrenergic hyperactivity is a major cause of psychiatric symptoms, and beta-adrenergic antagonists are effective treatment. Most patients with severe hypothyroidism will also demonstrate mental symptoms; however, causality is not so evident as in hyperthyroidism. Polymorphism in deiodinase genes and in transporter genes appears to make an important contribution to the presentation of mental symptoms as well as to the outcome of treatment of hypothyroidism. A thyroid autoimmunity process may by itself contribute to mental symptoms in vulnerable patients. Data from epidemiological studies provide conflicting evidence as to associations between thyroid disorders and mental symptoms.

In the adult brain, compared with the developing brain, brain-thyroid relationships are less apparent but still important. Adrenergic hyperactivity is a major cause of psychiatric symptoms in hyperthyroidism. Genetic factors contribute to the development and treatment outcome of mental disorder in hypothyroidism.