Thyroidal Iodide, Decreased leads to T4 in serum, Decreased

Human studies have used potassium perchlorate to study the perchlorate inhibition of thyroidal iodide uptake and the effects on thyroid homeostasis. These studies were mainly short-term exposure studies and they failed to predict the association between perchlorate and serum concentration of thyroid hormones (Greer et al., 2002; Brabant et al., 1992; Lawrence et al., 2001; 2001). Because humans have relatively large amount of iodinated thyroglobulin in the colloid of thyroid follicles they can produce thyroid hormones for up to several weeks, even in the absence of iodide uptake. Therefore the human studies with exposure durations of 2 weeks or less possibly cannot identify toxic effects that may occur for long exposure durations. Surprisingly, a 6-month exposure to perchlorate at doses up to 3 mg/d (low doses) had no effect on thyroid function, including inhibition of thyroid iodide uptake as well as serum levels of thyroid hormones, TSH, and Tg (Braverman et al., 2006). This study was limited by the small sample size, however it supports the notion that low dose perchlorate in the environment does not cause adverse effects in the thyroid.

Interestingly, in one of the studies that perchlorate was negatively associated with T4 that happened only in women with iodine <100 μg/L (Blount et al. 2006), suggesting that iodine levels must be sufficiently low for environmental levels of perchlorate and thiocyanate to overcome compensatory mechanisms that maintain thyroid hormone (Steinmaus et al. 2007) at least in adults. Finally, in vitro studies of NIS inhibitors indicate that perchlorate, thiocyanate and nitrate act additively to inhibit iodide uptake (Tonacchera et al. 2004), thus suggesting that these exposures should be considered in combination in order to obtain robust results, and that kind of studies are still lacking.