Effects of oestrogen on urinary thyrosine excretion

The day to day variation and the effects of oestrogen on the urinary excretion of thyroxine (T-4) were studied in euthyroid women and men. Serial urinary T-4 values over a period of 28 consecutive days were found to lie within relatively narrow limits except for a transient increase during menstruation in women. During oestrogen therapy urinary T-4 was unchanged, but an appreciable rise was seen after stopping oral ovulation inhibitors in women. A similar effect was seen in men after three days' treatment with 20 mug/day of ethinyloestradiol. The increased urinary T-4 excretion on oestrogen withdrawal reached a maximum in one to three days. This response contrasted with that produced by phenytoin, a drug known to bind to thyroxine binding globulin, and which resulted in increased urinary T-4 excretion during the period that it was being administered.