The thymus gland is found behind the sternum in the middle of the chest. It plays a significant role in the immune system, especially in unborn and very young children. The theory behind the use of thymus extracts is that they might stimulate or normalize immunity. However, there is no reliable real evidence as yet that any thymus extracts are effective for any health condition. Furthermore, there are significant safety concerns with thymus products.

Thymus extract is produced primarily from the thymus gland of cows. This has led to concerns regarding "mad cow" disease. All the studies described below used a pharmaceutical-grade form of thymus called Thymomodulin. It is not known whether the thymus supplements available as a dietary supplement would have the same effect.