When is unintentional weight loss considered critical?

Usually, other symptoms appear because the weight loss could be due to hyperthyroidism. If you are losing weight though you are eating and exercising the same amount as usual, it would be wise to get treatment from a doctor, since it could be a sign of a serious medical issue. A couple pounds here or there is not an issue, but a sudden loss of 8 or more pounds is.

Read more here: http://en.wikipedia.org/wiki/Weight_loss#Unintentional_weight_loss