Having a goiter that has grown over years could be sub sternal thyroid?

Possibly: It could be or it could just be an enlarging goiter. A thyroid u/s is the first test to evaluate this. Other tests can follow but you usually will know the seriousness of this with a thyroid ultrasound. Good luck.

Growth: Growth is concerning. Whether the gland grows sub sternally or into the neck is unpredictable. If there are no dominant or suspicious nodules( which should undergo fna biopsy), then it can be watched until there are obstructive symptoms (shortness of breath, swallowing problems, etc.).

Any symptoms?: Two issues to know if clinically significant: 1. Any nodules that look suspicious? Can check w/ ultrasound, and if yes, get biopsy.2. Causing any symptoms? Difficult breathing when lying flat, trouble swallowing, hoarse voice. If so, discuss with endocrine surgeon or ENT about removal. If very deep into chest (substernal), may need thoracic surgeon on board too.