Trigger finger is usually caused by swelling affecting 1 of the tendons that run along your fingers and thumbs, although the exact reason why this occurs is not fully understood.

The swelling and inflammation of the tunnel that the tendon runs through (tendon sheath) makes it difficult for the affected tendon to slide easily, causing the pain and stiffness associated with trigger finger.

The swelling can cause a section of the tendon to become bunched into a small lump (nodule) at the base of the affected finger or thumb.

If a nodule forms, the tendon can get trapped in the tendon sheath, causing the affected finger or thumb to become temporarily stuck in a bent position. The affected tendon may then suddenly break free, releasing your finger like the release of a trigger.

The exact cause of trigger finger is not fully understood. However, you're more at risk of developing trigger finger if:

• you're female (trigger finger is more common in women than men)
• you're in your 40s or 50s (trigger finger is more common in this age group)
• you've had a previous hand injury – trigger finger may be more likely to develop after injuring the base of your finger or palm

Other health conditions

You may also be more likely to develop trigger finger if you have any of the following conditions:

• diabetes – a lifelong condition that causes a person's blood sugar level to become too high
• rheumatoid arthritis – a long-term condition caused by a problem with your immune system (the body's natural defence system), resulting in pain and stiffness in your joints
• gout – a type of arthritis where crystals of sodium urate form inside and around joints causing them to become inflamed (swollen)
• amyloidosis – a condition where an abnormal protein called amyloid builds up in organs, such as your liver
• underactive thyroid (hypothyroidism) – where your thyroid gland does not produce enough of certain hormones
• carpal tunnel syndrome – a condition that affects the nerves in your wrist, causing pain and tingling
• Dupuytren's contracture – a condition that causes 1 or more fingers to bend into the palm of your hand
• De Quervain's disease – a condition that affects the tendons in your thumb, causing pain in your wrist