Peroxidase-inhibiting goitrogens

Thiocyanate ion, SCN , inhibits formation of thyroid hormones by inhibiting the iodination of tyrosine residues in thyroglobufin by thyroid peroxidase. This ion is also responsible for the goitrogenic effect of cassava (manioc, tapioca). Cyanide, CN , is liberated by hydrolysis from the cyanogenic glucoside finamarin it contains, which in turn is biodetoxified to SCN. 

Aromatic Amines and Phenols. The discovery that sulfaguanidine [57-67-0] was goitrogenic to rats was serendipitous. Many related compounds were then examined, and the aniline moiety was usually present (2,6). Such compounds, as well as resorcinol-like phenols, may act as goitrogens by inhibiting thyroid peroxidases. These are not used clinically. 

Results from several studies have raised speculations on the effect of isoflavones on thyroid function. Goitrogenic effects in infants fed soy-based IF were first reported in the 1960s due to increased loss of thyroxine from the gut and inhibition of thyroid peroxidase in the absence of iodine, a key enzyme in the production of thyroid hormone (Divi et al. 1997). The subsequent replacement of soy flour with SPI and supplementation of IF with iodine overcame the goitrogenic effects of soy-based IF (Merritt and Jenks 2004), and there are now no coneems regarding th Toid disease. Review of the literature suggests that there is little evidenee that thyroid function would be adversely affected by SF or SI supplementation in euthyroid, iodine-replete individuals (Messina... 

Perchlorates (see Section 6.3.2.2) and some degradation products of glucosinolates, such as goitrin, isothiocyanates, nitriles and thiocyanate (rhodanide) ions (SCN ), have goitrogenic (antithyroid) activities. Thiocyanate ions also arise in the body as detoxification products of cyanides. The other compounds that show antithyroid activity are some congeners of polychlorinated biphenyls (PCBs), some pesticides and a number of veterinary drugs containing thiourea residues (such as thiouracils, aminothiazoles and mercaptoimidazoles) that inhibit the enzyme thyroid peroxidase. These substances may be present in foods as contaminants (see Section 12.8.3.1). 

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