What are causes of pain in 3cm solitary benign thyroid nodule? and treatment?

Bleeding in nodule: Solitary nodules usually require needle biopsy. Most are benign and asymptomatic. I assume yours was diagnosed with a needle biopsy. Rarely there can be spontaneous bleeding into a cystic nodule, which can cause pain and is usually harmless.

Your Dr is the guide: You need to tell us more...Why did you go to your doctor to start with? What has he/she advised you to do?. The cause of pain is unknown. It can be bleeding into a cyst. Cancer cannot be excluded unless you get this thyroid nodule removed surgically.