Herniated disc in the thoracic spine is pretty uncommon and the reason for that is because there are a lot of supporting structures in the thoracic spine, unlike the cervical and lumbar spine. There is a lot more motion in the cervical spine as well as the lumbar spine. That motion is limited in the thoracic spine because of the rib cage. The rib cage is there to protect our internal organs, but by creating that rigid cage, it also limits - to an extent - the forces that are exerted through the actual disc. That being said, it certainly does occur.
To take a step back, it is important to remember what a herniated disc really is and it helps to visualize the disc as a jelly donut. Now, if you think of a jelly donut, that gelatinous center is the nucleus pulposus. That nucleus pulposus contains a number of different proteins that actually give it that gelatinous cushioning property. Those proteins can also be pro-inflammatory; in other words, causing inflammation. The outside - or the dough part of the donut - is composed of overlapping connective tissue belts and there are forty to seventy layers of those belts, depending on which literature you look at, but there is a considerable amount of that connective tissue that holds everything together. The important aspect of that is that - why do we not have pain and inflammation if we have those proteins just sitting there? Well, the reason for that is because they are contained and the nerves do not enter the inner two thirds of that outside connective tissue. However, the outside one third has rich innervation of little nerves that can sense pain. So, a herniated disc can mean several things. It is important to differentiate discogenic pain - in other words genesis of the pain is really from the disc itself as opposed to that disc compressing a nerve or irritating a nerve through leakage of those proteins due to rupture or inflammation. It is similar to stepping on a jelly donut and smooshing that gelatinous center out.
Discogenic pain usually presents with axial pain - in other words pain that remains in the spine - and that usually comes with a host of other symptoms like spasming muscles. Spasming muscles are really there to control motion. It is sort of a self-preservation. The muscles are there to control the motion so that we don't move what's going to hurt us, so that pain remains axial. If it irritates the nerve, the nerve itself can send the pain down its distribution or it can send the pain around the intercostal nerve or along the ribcage and it itself becomes more radicular in nature - or in other words, pain that radiates and pain that goes out from the center - as opposed to the axial pain.
Symptoms due to the herniated disc can vary, depending on what we're talking about whether its discogenic or pain that causes irritation and compression of the nerve. So, symptoms can vary from pain that is really localized to the spine, and that can be aching, deep type of a pain limiting motion and some people actually describe it as something they need to work out, kind of like a muscle knot they just can't get to it. Sometimes that pain can radiate around the ribcage. Sometimes it can actually feel like a deeper pain; pain that's in the back but deeper than the musculoskeletal component of it and that's because of the pain that really travels with the nerve or really is referred pain through that nerve. At times - because of the muscle tightness - it can actually affect the mobility and the motion and then the whole biomechanical chain starts crumbling down and then people tend to have pain that spreads and spreads more to the upper back and more to the neck and supporting muscles to the neck, the shoulder and to the lower lumbar spine.