The two main tendons that make up the shoulder are the biceps tendon and the rotator cuff tendons. Shoulder tendonitis occurs when there is an inflammation of either one of these tendons. 

There are two main types of shoulder tendonitis, these are:

• Rotator cuff tendonitis: inflammation of the rotator cuff tendon
• Biceps tendonitis: inflammation of the upper biceps tendon.

Both occur as a result of being pinched or squeezed by nearby structures such as bones.

• Injury: sports injuries are the most common cause of shoulder tendonitis but can also be caused by falling directly on the tendons. 
• Repetitive use or overuse: occurs due to sports, hobbies, or occupations that require repetitive movements or heavy lifting, thus putting excessive weight and strain on the tendons.

General symptoms of shoulder tendonitis include:

• Pain or discomfort in your shoulder which is more pronounced during movement
• Limited range of motion
• Swelling 
• Snapping sound or feeling in shoulder

• Occupation: certain occupations such as those that involve heavy lifting or repeated/awkward movements can cause your rotator cuff tendon or biceps tendon to become snagged or pinched by nearby structures. 
• Hobbies/sports: certain activities such as heavy lifting at the gym can cause irritation or inflammation of your shoulder tendons.
• Age: as we get older, our tendons start to lose their flexibility, making them more prone to injury.
• Medical conditions: diabetes can increase your risk of shoulder tendonitis.
• Medications: certain medications such as some antibiotics and corticosteroids can increase your risk of shoulder tendonitis.

For proper diagnosis, you may be required to undergo the following: 

• Physical examination: the first step of diagnosis will be a physical examination and a discussion on your medical history. This helps to identify the area of pain, range of motion, and possible cause of your shoulder tendonitis. 
• X-ray: an x-ray will identify the areas of irritation and inflammation.
• Ultrasound or magnetic resonance imaging (MRI): ultrasounds and MRIs will check for the presence of inflammation of the tendons and identify which tendons are inflamed. 

• Medication: pain relievers and non-steroidal anti-inflammatory drugs (NSAIDs) will be prescribed to alleviate your pain and reduce inflammation in your shoulder.
• Corticosteroid injections: these injections help to reduce inflammation and relieve pain but it cannot be used as a long-term solution– long term use of steroids can weaken tendons.
• Platelet rich plasma (PRP): PRP involves the use of platelets from your own blood to help in the healing process. 
• Physiotherapy: physiotherapy involves the use of therapeutic and gentle exercises to help strengthen your muscles and tendons.
• Surgery: minimally invasive Arthroscopy is usually used to treat shoulder tendonitis when other non-invasive treatment options have failed.