The phrase "an effect on thyroid" is not entirely correct in written English; it should be "an effect on the thyroid." You can use it when discussing the impact of something on thyroid function or health.
Example: "The medication was found to have an effect on the thyroid, leading to changes in hormone levels."
Alternatives: "an impact on the thyroid" or "a consequence for the thyroid."
Postnatal exposure cannot be excluded as an influence, but exposure from breast-feeding was not linked to an effect on thyroid hormones.
Further, recent evidence from the same NHANES/CDC data set described above indicates an effect on thyroid hormone status in adult women at perchlorate intake levels that are below the RfD (Blount et al. 2006a).
The potency of HCG for TSH receptors is some 4000 times less than TSH and hence, extremely high levels of HCG are usually required for an effect on thyroid function to be seen.
In addition, sulfation may decrease the compound's toxicity via disruption of binding, because in vivo studies of the sulfoconjugate of TBBPA demonstrate only the parent compound having an effect on thyroid hormone signaling (Fini et al. 2012).
Except for the decrease in FT4 in the latter study, these findings can be explained by an effect on thyroid function at the peripheral level causing a decreased level of circulating thyroid hormone and consequently an increase of TSH by negative feedback mechanisms.
Exposure models predict that infants are unlikely to be exposed to doses this high (Baier-Anderson et al. 2006); however, infants may be more sensitive than adults to thyroid inhibition (Lawrence et al. 2000), thus raising the possibility of an effect on thyroid function in infants from background exposure to perchlorate.
Further work is needed to determine whether such exposures have any effect on thyroid function.
Common variants in PDE8B are associated with TSH but apparently without any effect on thyroid hormone levels that is difficult to explain.
Because it is relatively easy to measure thyroid hormone concentrations, most studies to date use circulating levels of thyroid hormone as the primary indicator of an effect on the thyroid system, or focus on mechanisms by which contaminants could affect thyroid hormone levels.
Inhibition of iodine uptake is a step in this pathway, but compensatory mechanisms may prevent an effect on circulating thyroid hormone levels.
Obviously, this makes it very difficult to determine which of the compounds may actually have an effect on human thyroid hormone levels.
