The study also suggests that physicians could use indicators of thyroid function to predict patients' responsiveness to antidepressants. As inpatients with depression often undergo routine thyroid tests, the data that physicians would need to create such a prediction are likely to be available to them already.
Researchers from Greece studied 30 patients suffering from major depression, and 60 healthy people as controls. Each patient was examined by two psychiatric experts, who assessed their condition during a structured interview. The researchers then tested the thyroid function of all the volunteers.
Although the levels of the thyroid function indicators FT3, FT4 and TSH, fell inside the normal range for all the people studied, suffering from depression appeared to increase the level of thyroid binding inhibitory immunoglobulins in the blood of some patients. High levels of these immunoglobulins can subtly inhibit the function of the thyroid gland.
The authors write: "Although thyroid dysfunction is not common in depression there is evidence suggesting the presence of an underlying autoimmune process affecting the thyroid gland in depressive patientsThe finding that depression often co-exists with autoimmune subclinical thyroiditis suggests that depression may cause alterations in the immune system, or that in fact it is an autoimmune disorder itself."
Two years after the initial examination, the patients were re-assessed, to find out how well they had responded to treatment for their condition. The patients' responsiveness was associated with the levels of thyroid hormone and thyroid binding inhibitory immunoglobulin in their blood.