Since how long are your symptoms? Were any investigations done for this? Do you smoke? Are you diabetic? Are you on any medications?
An MRI of the lower back and bone scan would be essential with your history of papillary thyroid carcinoma, if they have not been done earlier. This can be due to degenerative disc disease, which most often occurs in the lower spine but can occur anywhere along the spinal column. With this condition, the cushions, or discs, between the vertebrae become smaller or disappear. This can occur due to loss of fluid in the disc, making is narrower or due to tearing of the disc causing bulging or herniation of the disc and its gel-like components into the surrounding area. In either circumstance, pain is the most common complaint. The pain may be in the back, neck, hip, buttocks or may radiate down the legs (sciatica). Over the years, bone spurs may develop as the body reacts to the smaller space between the vertebrae. These spurs can press on the spinal cord and cause pain and/or neuropathy. Neuropathy is pain, numbness and tingling. Sometimes this pain is severe. This is also called as pinched nerves. Read about it;
Apart from the MRI and bone scan in your case neurophysiological studies like electromyography and nerve conduction studies (EMG & NCV) will be required to substantiate the diagnosis.
Please feel free for your follow up questions.
A long history of three months with pain in back of thigh radiating to knee and calf, knee feeling thick when bending and painful and uncomfortable even on sitting, points to pinched nerves or radiculopathy. The other differential diagnosis is knee osteoarthritis. In your age it is not common and knee osteoarthritis is usually not painful when one is sitting.
A clinical examination by a physician gives clear idea and clue to the diagnosis and differentiates between the two.
It is pleasure assisting you.