Abstract :
In sheep, phytoestrogens are known to act at various levels on the hypothalamo–
pituitary–gonadal axis, as well as the hypothalamo–pituitary–thyroid axis. The levels
of genistein and daidzein and their metabolites, p-ethylphenol and equol, in the cerebrospinal
fluid (CSF) were studied in ewes fed red clover silage. Moreover, to test the
hypothesis that phytoestrogens may affect the access of thyroid hormones in the brain,
the concentrations of both total (TT4) and free thyroxine (fT4) in the blood and CSF,
were recorded. In the CSF, daidzein, equol and p-ethylphenol were present, but genistein
absent. The total daidzein concentration increased from an undetectable level at 2 h,
to 0.11 ± 0.05 M at 6 h after feeding, representing 13.1% of the daidzein measured in the
blood. Gradual increases in equol, from an undetectable level at 2 h, to 0.24 ± 0.13 M at
6 h after feeding (5.1% of that in blood), and in p-ethylphenol, from 0.03 ± 0.02 M at 2 h to
1.7 ± 0.74 M at 6 h after feeding (0.1–5.8% of that in blood) were recorded. In sheep fed a
phytoestrogen rich diet, the blood concentration of TT4 was lower (P < 0.05), but the concentration
and the percentage of free T4 (fT4) higher (P < 0.01) than in the control sheep. In
the CSF, the TT4 concentration was similar in both groups, but the concentration and percentage
of fT4 was higher in sheep fed a phytoestrogen rich diet. It could be assumed that (i)
in sheep, reproductive disorders, at the base of the hypothalamus, following phytoestrogen
consumption are evoked by daidzein and equol, its metabolite, rather than genistein, and
(ii) phytoestrogens affect the brain’s ability to access T4 in the blood.
