Yes, I agree Anke, this is what I understand:
Pregnancy toxaemia/twin lamb disease is an energy imbalance, ie the mother goat can't keep up with the energy demands of her kids, so she uses up her own stores of fat. Using fat produces ketones. They reduce appetite, which is the wonderful thing in low-carb dieting, you don't feel too hungry. But for a goat it can result in a vicious circle, they can't meet their kids' energy needs, they break down fats to do so, this reduces their appetites, they don't eat, therefore they can even less meet their kids' demands......
Cuckoo, I think you use your judgement about your goat - my feeling is that we usually have a sense of what's right for our animals and if yours is that your goat should be retired from breeding, you're probably right. Of course older goats can breed successfully - but maybe not ones that have suffered problems in the past.