Please read the page on thyroid goiters before you read this page. It will make it easier to understand. The normal thyroid gland resides in the neck, with both lobes wrapping gently around the trachea (breathing tube).
The normal thyroid gland resides in the neck, with both lobes wrapping gently around the trachea (breathing tube). When thyroids get enlarged (called a goiter), they can grow a number of different directions. Usually, they will grow within the neck as shown in the picture of the woman on our goiter page. When this is the case, they are seen as a large mass in the neck. Since they grow slowly, taking a number of years to obtain their large size, many people aren't aware of just how large the thyroid has become.
Less commonly, a thyroid will grow downward rather than up and out within the neck. When this happens, the thyroid will grow down the trachea into the chest. This can become an even bigger problem since the chest is surrounded by a very rigid bone structure (the chest cavity). The top of the chest cavity is made up of the spinal column in the back, the first and second ribs on the sides, and the collar bones (clavicles) and breast bone (sternum) in the front. When a thyroid gets enlarged within this rigid bony structure, it will compress those structures which are soft such as the trachea, lungs, and blood vessels (the bones will not give way). This is what makes sub-sternal thyroids a special case which deserve special attention.
This chest x-ray shows a sub-sternal thyroid which is compressing and displacing the trachea to the patient's left The trachea (outlined in light yellow) should run straight from the mouth/nose down to the lungs rather than being curved like it is in this picture. This is not as un-common as you may think.
This CT scan shows a similar problem. The thyroid goiter (outlined in yellow) has grown into the chest below the sternum. The trachea (outlined in red) is displaced to the patient's right side (shown on CT scans on the left of the picture). Remember, the trachea is supposed to be in the middle of the chest and the thyroid should not extend into the chest at all. This patient has a hard time swallowing breads and meats, and she feels like she is suffocating when she lies on her back.
It is a misconception that all sub-sternal thyroids require that the sternum be split to allow it to be removed. In fact, this is extremely rare. Essentially all sub-sternal thyroids can be removed through a conventional thyroid neck incision. It must be remembered that the blood supply to the thyroid is from two separate sources both of which arise in the neck (and not the chest). That means that the blood supply can be cut off from above without undue fear of intra-operative bleeding. After the patient is put to sleep under anesthesia, his/her neck can be extended fairly far backward which helps pull the thyroid up from the chest making it easier to remove. Remember, even though these goiters can extend WAY down into the chest, it is very uncommon for a sternal splitting operation to be necessary.