While TPP is found in people of all ethnicities, it is most common in Asians, followed by Latinos, Caucasians and people of African descent, and is far more common in men than women.

While he couldn't ethically justify doing the study, says Ptacek, he predicted that if all the unaffected people in families of TPP patients were treated with high doses of synthetic thyroid hormone, the process would unmask a familial pattern of TPP. (In practice, TPP would still be considered a sporadic condition in most cases, since high thyroid hormone levels usually don't affect more than 1 or 2 people from any family in whom a mutated gene is present.)

In any case, he says, the finding exemplifies how study of familial, or inherited, forms of a disease can sometimes lay the groundwork for understanding more common non-familial, or sporadic, forms.

"Identifying the role of specific genes in complex diseases such as the sporadic form of Alzheimer's disease, Parkinson's disease and autism has proven challenging," says Ptacek. "Here's a case where we were able to identify the gene underlying a sporadic disease, by first understanding the rare familial forms."

The finding also illustrates the power of one human genetics discovery to fuel another - and possibly another yet. In his 1991 paper (Cell. 1991 Nov 29;67:1021-7) describing the role of ion channel mutations in familial periodic paralysis, Ptacek predicted that mutations in ion channel genes of heart muscle cells could be the cause of electrical alterations in Long-QT syndrome, a rare congenital heart arrhythmia that can be fatal, and that mutations in ion channels in the brain could be the cause of genetic forms of epilepsy and migraine. The predictions turned out to be true.

In the current paper, he suggests that the reason thyrotoxic patients frequently develop cardiac arrhythmias may be because they have genetic mutations in ion channels of heart muscle cells that are regulated by thyroid hormone. Time, and research, he says, will tell.