Increased production of thyroxine and inappropriately elevated serum thyrotropin in levels in endogenous depression

The increased serum thyroxine (T4) levels in endogenous depression (ED) might be due to an increased production or a reduced degradation of T4. We therefore performed turnover studies of radiolabeled T4 and 3,5,3'-triiodothyronine (T3) in 6 patients with ED and 8 age-matched healthy controls. In ED, the median daily production of T4 was 130 nmol/day/70 kg, (range 100-186 nmol/day/70 kg) and elevated compared to control values which were 99 nmol/day/70 kg (range 85-142 nmol/day/70 kg) (p less than 0.05), whereas that of T3 was similar in the two groups. Serum thyrotropin (TSH) levels (0.90 mU/liter, 0.18-2.15 mU/liter) were elevated in ED compared to a group of 7 L-T4-treated hypothyroid subjects with similar production and serum levels of T4 and T3 (0.11 mU/liter, 0.07-1.10 mU/liter) (p less than 0.02). The data show that increased serum T4 levels in ED are secondary to an increased thyroidal production of T4, which is at least partly due to inappropriately high serum TSH levels.