CNW reports that hyperthyroidism might contribute to some ecstasy deaths.

 [A] young woman [...] developed fatal hyperthermia after taking ecstasy. It was later discovered at autopsy that she had an overactive thyroid (hyperthyroidism), a condition that can make some people less tolerant to heat. 

[...] supported by other data showing that experimental animals that are hyperthyroid are more likely to die when exposed to ecstasy.