Additive-like effects have been observed from multiple thyroid hormone disruptors given to rats by oral gavage for 4 consecutive days (Devito et al., 2003). These disruptors are dioxins, furans, and dioxin-likes such as PCDDs, PCDFs, and PCBs. Combinations of these disruptors were shown to aggravate a decrease in serum thyroid hormone level, which can lead to hypothyroidism involving fatigue, depression, confusion, and other symptoms.

Another example of cumulative effects on thyroid hormone disruption is the five-year study by Porter et al. (1999). In that study, synergistic effects were observed in mice of two species after the test animals had been dosed with mixtures of aldicarb (an insecticide), atrazine (a herbicide), and nitrate (a fertilizer). These pesticides were given to the animals at levels within the range often encountered in groundwater in the USA. The low-level mixtures of either nitrate with atrazine or of all three pesticides were seen to increase thyroid hormone levels that changed the test mice’s aggression toward intruder mice. Hyperthyroidism, a condition caused by the effects of too much thyroid hormone on body tissues, has been known to associate with irritability, aggression, nervousness, and other symptoms.

Cumulative effects of this type are highly critical in that thyroid functions are important for proper growth, mental development, metabolism, and reproduction. Thus pregnant women, developing fetuses, and infants are especially vulnerable to adjustments in thyroid hormone levels. Several studies (e.g., Allan et al., 2000; Morreale de Escobar et al., 2000; Klein et al., 2001) indicated that imbalances of thyroid hormones in pregnant women can cause some neuropsychological damage in the developing fetus, including disabilities of learning.