Many goitrogenic xenobiotics that increase the incidence of thyroid tu
mors in rodents exert a direct effect on the thyroid gland to disrupt
one of several possible steps in the biosynthesis, secretion, and meta
bolism of thyroid hormones. This includes (a) inhibition of the iodine
trapping mechanism, (b) blockage of organic binding of iodine and cou
pling of iodothyronines to form thyroxine (T-4) and triiodothyronine (
T-3), and (c) inhibition of thyroid hormone secretion by an effect on
proteolysis of active hormone from the colloid. Another large group of
goitrogenic chemicals disrupts thyroid hormone economy by increasing
the peripheral metabolism of thyroid hormones through an induction of
hepatic microsomal enzymes. This group includes central nervous system
-acting drugs, calcium channel blockers, steroids, retinoids, chlorina
ted hydrocarbons, polyhalogenated biphenyls, and enzyme inducers. Thyr
oid hormone economy also can be disrupted by xenobiotics that inhibit
the 5'-monodeiodinase that converts T-4 in peripheral sites to biologi
cally active T-3. Inhibition of this enzyme by FD&C Red No. 3 lowers c
irculating T-3 levels, which results in a compensatory increased secre
tion of thyroid stimulating hormone (TSH), follicular cell hypertrophy
and hyperplasia, and an increased incidence of follicular cell tumors
in 2-yr or lifetime studies in rats. Physiologic perturbations alone,
such as the feeding of an iodine-deficient diet, partial thyroidectom
y, natural goitrogens in certain foods, and transplantation of TSH-sec
reting pituitary tumors in rodents also can disrupt thyroid hormone ec
onomy and, if sustained, increase the development of thyroid tumors in
rats. A consistent finding with all of these goitrogens, be they eith
er physiologic perturbations or xenobiotics, is the chronic hypersecre
tion of TSH, which places the rodent thyroid gland at greater risk to
develop tumors through a secondary (indirect) mechanism of thyroid onc
ogenesis associated with hormonal imbalances.