Abstract
Objectives
There is increasing concern that exposure to flavonoids may lead to endocrine disruption
of the hypothalamus–pituitary–thyroid hormone axis, and, additionally, there is evidence
that secondary plant metabolites contained in our daily diet or used for hormone-replacement
therapy act as hormones themselves, similar to known isoflavonoid effects in the steroid
hormone network. These compounds of natural origin affect the thyroid hormone feedback
system by interference with different components of this homeostatically regulated
system: biosynthesis, secretion and metabolism, transport, distribution, and action
of thyroid hormones including the feedback mechanism. Genistein and daidzein, the
major components of soy, influence thyroid hormone synthesis by inhibition of the
iodide oxidizing enzyme thyroperoxidase, interfere with thyroid hormone transport
proteins and 5′-deiodinase type I activities in peripheral tissues, which leads to
altered thyroid hormone action at the cellular level. Synthetic flavonoids, such as
F21388, structurally similar to thyroxine, cross the placenta and reach the fetal
brain of animal models also.
Methods
A review of effects of various isoflavonoids and plant-derived extracts on the hypothalamus–pituitary–thyroid
axis is the major objective of this contribution. In addition, new experimental data
obtained in ovariectomized (ovx) rats will be presented. The substances tested here
were plant extracts of Agnus castus, Belamcanda chinensis, Silybum marianum, Cimicifuga racemosa, and a commercially available soy product.
Results
In ovx rats the extract at two doses showed no effects on circulating TSH and thyroid
hormone serum levels after 3 months of treatment.
Conclusion
The thyroid hormone network has a considerable capacity to compensate for disturbances
of this feedback system as it is necessary for various metabolic and catabolic reactions
and development. With respect to still insufficient nutritive iodine-supply of almost
one third of the world population, possible adverse flavonoid actions on the thyroid
hormone axis have to be examined more closely.
Keywords
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Article info
Publication history
Published online: July 20, 2006
Identification
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© 2006 Elsevier Ireland Ltd. Published by Elsevier Inc. All rights reserved.
