BIOMEDICAL ASPECTS OF THIMEROSAL EXPOSURE
Submitted to the IOM 2004 committee on Autism
by
BOYD E. HALEY, PROFESSOR AND CHAIR, DEPARTMENT OF CHEMISTRY, UNIVERSITY OF KENTUCKY 

FORWARD: Thimerosal or merthiolate is a derivative of thiolsalicylate where ethyl-mercury is attached though the sulfur or thiol group. It is defined as a preservative or anti-microbial in medical use. This anti-microbial action is dependent on thimerosal breaking down releasing ethyl-mercury that can penetrate cell membranes and bind to intracellular enzymes, inhibiting them, and causing cell death. Further, in certain biological environments the ethyl-mercury can further break down releasing mercury cation (Hg2+). Hg2+ is also very reactive with enzymes and proteins inhibiting their biological functions and causing cell injury or death.
Ethyl-mercury, as it exists in a biological system, is more rapidly partitioned into the hydrophobic (fatty) tissues of the central nervous system and is a more potent neuro-toxin than Hg2+ based on this “partitioning factor”. It is this partitioning factor that makes organic-mercurials such as dimethyl-mercury so neuro-toxically lethal (this is the compound that caused the death of a Dartmouth University chemistry professor after she was exposed to a drop or two on her gloved hand). The concern with organic-mercurials, such as thimerosal, is that such compounds can be perceived as “pro-toxicants” just as certain pharmaceuticals can be classified as “pro-drugs”. This means that the original compound, e.g. thimerosal, is less reactive giving the compound time to partition into certain areas of the body before it breaks down releasing the ethyl-mercury and then further releasing Hg2+. However, it is not necessary for ethylmercury to break down to Hg2+ to be toxic. Ethylmercury appears to be more toxic to enzymes than Hg2+ in testing systems where conversion of ethylmercury to Hg2+ would be extremely slow.
Considerable caution must be taken when stating what is the “toxic level” of mercury and any mercury containing compounds. Humans are not rats in a pristine cage where their environment can be controlled to ensure that other toxicities and infections are not occurring. The level of mercury that would cause toxicity in a healthy individual is much higher than what would be needed to cause a toxic effect in an individual that is ill or under oxidative stress. This is because additional stresses lower the amount of protective compounds, such as glutathione, that bind mercury and render it less harmful and aid in the excretion of mercury from the cells. If an individual is low on such protective compounds, then less mercury or thimerosal would be needed to cause a clinical effect.
Additionally, it is well known that many toxicants have corresponding compounds that will enhance their toxicity if present at the same time. For example lead is well known to enhance the toxicity of mercury. Also, the antibiotic, tetracycline, enhances the toxicity of thimerosal in ocular tissue. We have observed that several compounds or metal ions enhance the toxicity of thimerosal when tested using neurons in culture. Included are the effects of estradiol and testosterone on thimerosal toxicity against neurons in culture that may help explain the 4:1 boy to girl ratio seen in autistic children. This latter observation could be expanded to a consideration of many complex cyclic organic compounds that may have an enhancing effect on mercury toxicity. It is the synergistic enhancement of toxicity of thimerosal-based toxicity that must be considered when evaluating children for apparent increased susceptibility to adverse effects to thimerosal exposure.
A recent publication from Denmark indicate that thimerosal present results that “do not support a causal relationship between childhood vaccination with thimerosal-containing vaccines and development of autistic-spectrum disorders.” (A. Hviid et al. Association Between Thimerosal-Containing Vaccine and Autism. JAMA v290, #13, 1763-1766). This was preceded by another Danish study which concluded that “The discontinuation of thimerosal-containing vaccines in Denmark in 1992 was followed by an increase in the incidence of autism (K.M. Madsen et al. Thimerosal and the Occurrence of Autism: Negative Ecological Evidence from a Danish Population-Based Data. Pediatrics v112#3, 604-606, 2003). Overall, these papers indicate that exposure to a potent neurotoxin reduces the incidence of a neurological disease, which seems most unlikely. Looking at the data in Figure 1 of Madsen et al. gives one severe concerns regarding the thimerosal/autism issue. They show an average incidence rate of less than 0.3-0.4 per 10,000 before 1990 and a maximum incidence rate of much less than 5 per 10,000 in 1999-2000. This later rate is close to what existed in the USA before 1980, before our autism rates started soaring. It is also well known that the Danes do not vaccinate their children with thimerosal vaccines on their day of birth, they do it 5 weeks later and only use half the mercury dose infants in the USA received on their first vaccination. Additionally, in the USA the first vaccination was also followed by a more aggressive vaccine scheduled that subjected infants to a much higher total exposure to thimerosal. Perhaps the autism rates in Denmark versus the USA should be considered as objective proof that earlier and increased exposure to thimerosal via vaccinations is causal for autism. It would make more sense. For certain, it is not logical to compare the effects of thimerosal on autism incidence in a country with the low Danish rate to that of the USA which is less than 5 versus over 60 per 10,000, respectively.
The observation of very low levels of mercury in the birth hair of autistic children in comparison to control, normal children is not to be lightly dismissed. Especially since the severity of the illness increased as the level of birth hair mercury decreased. This implies that autistic children do not biochemically or physiologically handle mercury excretion as do control children. It is my opinion, that the lack of mercury in the hair of autistic children is a reflection of the inability to excrete mercury from their cells. The observation of increasing birth hair mercury in normal children with increasing amalgam fillings in the birth mother, and the total lack of this with autistic children, suggests that the autistic children represent a subset of the population that cannot effectively excrete mercury from their cells. This inability may be due to genetic susceptibility factors that lead to increased retention of mercury. It may also be due to the presence of synergistic factors that interfere with mercury excretion (such as certain antibiotics), or both.
Below I will present my interpretation of our research and that from other laboratories that focus on the potential toxicity of injected thimerosal in the vaccine mixture.