Rotator Cuff Tendinitis or Tear: This condition is a common cause of shoulder pain and weakness, especially in individuals who engage in repetitive overhead activities or have experienced trauma to the shoulder. The presence of pain in both shoulders suggests a possible overuse or degenerative condition.

Cervical Spondylosis: This condition involves wear and tear on the discs in the neck, which can cause pain and numbness in the shoulders and arms due to compression of the nerves. The intermittent numbness in the left shoulder could be indicative of nerve root compression.
Adhesive Capsulitis (Frozen Shoulder): Characterized by stiffness and pain in the shoulder joint, adhesive capsulitis can lead to weakness and limited mobility. It often affects one shoulder but can occur in both.
Thoracic Outlet Syndrome: This condition involves compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib). It can cause pain, numbness, and weakness in the shoulders and arms.

Multiple Myeloma or Other Cancers: Although less common, cancers such as multiple myeloma can cause bone pain, including in the shoulders, and should not be overlooked, especially in older adults or those with risk factors.
Polymyalgia Rheumatica (PMR): This inflammatory condition causes muscle pain and stiffness, typically in the shoulders and hips. It's essential to diagnose PMR because it can be associated with giant cell arteritis, a condition that can lead to blindness if not treated promptly.
Stroke or Transient Ischemic Attack (TIA): While less likely to present with isolated shoulder symptoms, a stroke or TIA can cause sudden weakness or numbness. Given the potential for severe consequences, these conditions must be considered, especially if there are accompanying neurological symptoms.

Amyotrophic Lateral Sclerosis (ALS): A progressive neurodegenerative disease that affects nerve cells in the brain and the spinal cord, causing loss of muscle control. ALS is rare but should be considered in cases of progressive muscle weakness without a clear cause.
Brachial Plexitis (Neuralgic Amyotrophy): An inflammatory condition that affects the nerves of the brachial plexus, causing severe pain and weakness in the shoulders and arms. It is relatively rare and can be idiopathic or associated with other conditions.
Eosinophilia-Myalgia Syndrome: A rare condition associated with the ingestion of contaminated L-tryptophan supplements, characterized by muscle pain, weakness, and elevated eosinophil counts. Given its rarity and the specific context required for diagnosis, it is less likely but should be considered in the appropriate clinical setting.