CONGENITAL HYPOTHYROIDISM RATE IN PERCHLORATE CONTAMINATED AREA

I just received the final copy of the U.S. Public Health Service intial health consultation for perchlorate exposure in Rancho Cordova, California. It ends up the congenital hypothyroidism rate in Rancho Cordova's perchlorate-exposed zip code during the likely period of exposure (1990-1996) was 1 case / 1300 births, where the national average is 1/4000. The rate for the state of California in 1980-1989 was 1/3762, while in 1990-1996 it was 1/2842.  Approximately one half of all infants currently being born in California are Hispanic. It has always been presumed California's higher rate was due exclusively to the large Hispanic population, because nationally Hispanics have a higher congenital hypothyroidism rate that is supposed to be at least partly genetic in origin. Given the widespread distribution of low-level perchlorate contamination (5 ppb)  in Southern California and Arizona, and the even higher exposure rates (100 ppb) in Hispanic portions of Los Angeles, it's clear that the genetic assumption needs to be re-examined.

In the period 1980-1989, the congenital hypothyroidism rate for Hispanics in California was 1/2808, comparable to the present 1/3000 thyroid birth defect rate for Hispanics in Texas. Asians generally have the same thyroid birth defect rate as whites, (1/4000 - 1/5000) and blacks generally have a much lower rate of congenital hypothyroidism (1/10,000 - 1/30,000).  Note that the California-wide rate in the 1990s approximates the Hispanic rate of the 1980s.

Broadly speaking, the exposure to perchlorate in Southern California should reflect income and latitude: the lower the income, the more likely a family would drink tap water, while the lower (more southerly) the latitude, the more likely
perchlorate-tainted Colorado River water would be used. The alternative perchlorate-free water source is Sierra Nevada runoff, which enters Southern California from the north via the San Fernando Valley and San Bernardino County.  There are two major perchlorate groundwater plumes in Southern California: the San Gabriel River plume, which supplies a densely populated Hispanic area in Los Angeles County, and the Santa Ana River plume in San Bernardino County.  The core of the San Bernardino plume is used for agriculture rather than drinking water supply, and Riverside County receives a significant portion of its water from an aqueduct connected to a well field in the San Gabriel mountains.

In 1996-1997 the county-wide congenital hypothyroidism (CH) rates for Southern California were:

County       Cases CH / Births     Ethnicity of Total Population

Los Angeles      1 / 2492            (37% Hispanic, 11% Black)

Orange             1 / 2530             (23% Hispanic, 2% Black)

San Diego         1 / 2686            (20% Hispanic, 6% Black)

San Bernardino 1 / 3037             (26% Hispanic, 8% Black)

Riverside           1 / 3961            (26% Hispanic, 5% Black).

Note that the southern counties of Orange and San Diego have high rates of thyroid birth defects that exceed the 1980s Hispanic rate, and their ethnic mix is roughly comparable with San Bernardino and Riverside counties.

Elsewhere, the thyroid birth defect rate for Sierra-runoff-supplied Sacramento County is 1/4904 (Total population 11% Hispanic, 9% Black) The potentially responsible parties for perchlorate contamination cite Las Vega's recent thyroid birth defect rate of 1/5145 as proof that low levels of perchlorate have no adverse health effects. However, the past  level of perchlorate contamination in Las Vegas is much less predictable than in  Southern California, because the level of ClO4- in the Vegas water intake depends on the drift of the currents in Lake Mead.  Since last summer Vegas water has been perchlorate-free because of these currents, whereas the well-mixed waters of the Colorado river provide a steady dose of ClO4- for Southern California.

It should be kept in mind that the recent increase in congenital hypothyroidism statistics in California may be due to factors such as improved screening or false positives from earlier testing times. No conclusions can be reached from such large aggregates of data, but intial inspection does invite a more detailed analysis. On the other hand, if congenital hypothyroidism is linked to the presence of the anti-thyroid peroxidase antibody in the mother as some researchers claim, then a change from a 1/4000 thyroid birth defect rate to a 1/2000 rate might reflect an increase in anti-TPO frequency from 5% to 10% of the childbearing population.
   
Larry Ladd

Community Representative
Aerojet Health Assessment Site Team
Rancho Cordova, California