The NAS Perchlorate Review: Adverse Effects?
To the editor: Drs. Ginsberg and Rice argue that the reference dose for perchlorate of 0.0007 mg/kg per day recommended by the National Academies’ Committee to Assess the Health Implications of Perchlorate Ingestion is not adequately protective. As members of the committee, we disagree. Ginsberg and Rice base their conclusion on three points. The first involves the designation of the point of departure as a NOEL (no-observed-effect level) versus a LOAEL (lowest-observed-adverse- effect level). The committee chose as its point of departure a dose of perchlorate (0.007 mg/kg per day) that when given for 14 days to 7 normal subjects did not cause a significant decrease in the group mean thyroid iodide uptake (Greer et al. 2002). Accordingly, the committee considered it a NOEL. Ginsberg and Rice focus on the fact that only 7 subjects were given that dose, and they 1seem to say that attention should be paid only to the results in those subjects in whom there was a 1fall in thyroid iodide uptake, and that the results in those in whom there was no fall or an increase should be ignored. They consider the dose to be a LOAEL because of the fall in uptake in those few subjects. It is important to note that a statistically significant decrease of, for example, 5% or even 10%, would not be biologically important and, more important, would not be sustained. For example, in another study (Braverman et al. 2004), administration of 0.04 mg/kg per day to normal subjects for 6 months had no effect on thyroid iodide uptake when measured at 3 and 6 months, and no effect on serum thyroid hormone or thyrotropin concentrations measured monthly (inspection of Figure 5A in the paper by Greer et al. suggests that this dose would inhibit thyroid iodide uptake by about 25% if measured at 2 weeks). The second issue involves database uncertainty. In clinical studies, perchlorate has been administered prospectively to 68 normal subjects for 2 weeks to 6 months. In one study (Brabant et al. 1992), a dose of 9.2 mg/kg per day for 4 weeks had no effect on thyroid function. In occupational studies, doses as high as 0.5 mg/kg per day were not associated with adverse effects on thyroid function in workers. In epidemiologic studies, there were no abnormalities in growth or thyroid function in children exposed life-long to 100 to 120 mg of perchlorate per liter of drinking water, or in pregnant women and newborn infants similarly exposed. Given the choice of a non-adverse effect (inhibition of iodide uptake by the thyroid) as the point of departure and the multiple studies in which doses of perchlorate much larger than 0.007 mg/kg per day had no effect on any aspect of thyroid function, the committee did not apply a database uncertainty factor. Finally, Ginsberg and Rice argue that inhibition of thyroid iodide uptake is adverse. That conclusion assumes that any acute inhibition would be sustained, so that thyroid hormone production would fall. That is not the case. There is remarkable compensation for even substantial reductions in thyroid iodide uptake – and thyroid hormone production. As noted above, subjects given 0.04 mg/kg per day for 6 months and 9.2 mg/kg per day for 4 weeks-doses that certainly would inhibit thyroid iodide uptake for a few weeks-had no fall in serum thyroid hormone or rise in serum thyrotropin concentrations (the hallmark of even minor systemic thyroid deficiency). Short-term inhibition of thyroid iodide uptake is not an adverse effect; it has no adverse consequences, because there is rapid compensation mediated by several independent processes. One is upregulation of the thyroid sodium-iodide transport system, as a result of intrathyroidal iodide deficiency. The second, should there be even a very small fall in thyroid hormone production, is an increase in thyrotropin secretion, resulting in overall stimulation of the thyroid gland.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 993669
- Report Number(s):
- PNNL-SA-47919; TRN: US201023%%515
- Journal Information:
- Environmental Health Perspectives, Vol. 113, Issue 11; ISSN 0091-6765
- Country of Publication:
- United States
- Language:
- English
Similar Records
Suppression of thyroid radioiodine uptake by various doses of stable iodide
Carbamazepine (Tegretol) inhibits in vivo iodide uptake and hormone synthesis in rat thyroid glands