Sweetbread is the name of a dish made of the thymus gland or pancreas of an animal younger than one year old. These animals are usually lambs or calves. According to Four Story Hill Farms, no American slaughterhouse has processed pancreas for chefs in over forty years. Larousse, published in Europe, also says the pancreas is rarely used. The pancreas is often sold to the pharmaceutical industry.
The two organs have very different biological functions, but look fairly similar and so are considered, for the purpose of cooking, to be comparable. Thymus sweetbreads are slightly longer and more irregular, with pancreas sweetbreads being larger and more rounded.
However, the thymus gland or European Sweetbread should not be confused with the American 'Restaurant' Sweetbread. The American variety, popularized by many 'scene' restaurants in New York and Los Angeles, are the testicles of sexually mature bulls or sheep. These delicacies are then floured, seared in a hot pan and served rare, making for a delicious appetizer. With an initial bite like chicken, the sweetbreads quickly melt away in the mouth with a texture not unlike foie gras. The testes are humanely extracted from the animal, normally with a lasso device, with the animal then living to average slaughter age with a supplemented diet to balance the deficiency in male hormone.