What is the difference between hyperextending & tendonitis below the thumb?

Different issues: Hyperextending the thumb really means that the thumb was straightened beyond its normal limit, causing a sprain of the plate on the palm-side of the joint. Tendonitis below the thumb is usually along the wrist and is called "dequervain's" tendonitis.It can be very painful and rarely goes away on its own. It can be treated. Earlier you get treated, the less likely it is that surgery will be needed.