THE HYPOTHALAMIC-PITUITARY-THYROID AXIS IN PATIENTS MAINTAINED ON LITHIUM PROPHYLAXIS FOR YEARS - HIGH TRIIODOTHYRONINE SERUM CONCENTRATIONS ARE CORRELATED TO THE PROPHYLACTIC EFFICACY

Serum concentrations of thyrotropine (TSH), thyroxine (T4), free T4 (f T4), triiodothyronine (T3) and reverse T3 (rT3) were measured 4x durin g a 12-month period in 28 patients with major depressive disorder main tained on lithium prophylaxis for 4-23 years (mean = 11.8). The course of illness was carefully monitored and documented for all patients th roughout a 3.5-year period. All hormones were also measured in 41 heal thy controls matched for age and gender. Patients on lithium had norma l serum concentrations of TSH, T4, fT4 and T3 only the levels of rT3 w ere elevated. The efficacy of the lithium prophylaxis was significantl y correlated to the serum concentrations of T3, i.e., the higher the p atients' serum levels of T3, the shorter was the overall duration of r ecurrences of depression within the 3.5-year period. We conclude that: (1) thyrotropine and the thyroid hormones, which are often abnormal d uring the first weeks or months of lithium treatment, returned to norm al when lithium prophylaxis was maintained for years; (2) a possible e xplanation for the higher T3-serum concentrations in responders might be that lithium interacts with thyroid hormone metabolism in the CNS, leading to enhanced T3 concentrations in the tissue and to a secondary increase in the serum concentrations of T3.