MELATONIN AND THE PITUITARY-THYROID AXIS STATUS IN BLIND ADULTS - A POSSIBLE RESETTING AFTER PUBERTY

OBJECTIVE Increased levels of free thyroid hormones have been previous ly described in prepubertal blind subjects and have been thought to be a consequence of a partial target organ refractoriness due to the ear ly and prolonged lack of light perception. The aim of this study was t o clarify whether this abnormality is permanent or transient and the i nterrelationships between melatonin and thyroid hormone secretion. MEA SUREMENTS Total and free thyroid hormones, TSH, thyroxine-binding glob ulin (TBG), reverse triiodothyronine (rT3) and melatonin were measured in plasma samples obtained at 0800 h (two hours after lights-on) in a group of 11 totally (group 1) and 16 partially (group 2) blind adult patients and in 10 age-matched healthy subjects. RESULTS Both totally and partially blind patients showed melatonin levels higher than in co ntrols (330 +/- 106 pmol/l, group 1 and 361 +/- 159 pmol/l, group 2, r espectively; controls: 53 +/- 12 pmol/l, P < 0.001 vs both groups), bu t fT4, fT3, T4, T3 TSH, rT3 and TBG concentrations showed no significa nt differences from controls. CONCLUSIONS A possible resetting of pitu itary-thyroid axis regulation can occur in blindness after puberty; va riations of melatonin secretion could play a role in this. The inhibit ory effect of melatonin on thyroid gland function found in animals doe s not seem to occur in humans. Elevated melatonin levels, both in pati ents with total blindness and in those with light perception only, sug gest that more complex mechanisms other than light signalling are invo lved in the changes of melatonin secretion in blindness.