As the report says, you have an enlarged thyroid gland, consistent with multinodular goiter.
If you live in a country where iodine is not added to salt and other foods, a goiter may be caused by an iodine deficiency. In countries where iodine is added to salt and other foods (like in the US), multinodular goiter is most often caused by an autoimmune disease, either Hashimoto's or Graves disease. Hashimoto's frequently causes hypothyroidism (and sometimes people experience hyperthyroidism symptoms as well), Graves disease can cause hyperthyroidism. Have your doctors tested your hormone levels to better understand what is causing your goiter?
Back to the report:
Substernal extension means part of the goiter extends below the neck and under the sternum - this can be a problem because the sternum and clavicles are bones that form a rigid structure, and if the thyroid extends beneath them it can compress other organs like the lungs and trachea.
The thyroid dimensions (thyroid is a butterfly shaped gland with left and right lobes and an isthmus in between) show that your thyroid is enlarged. Based on "resultant significant narrowing of the proximal tracheal airway at the level of T1-T2", it sounds like your thyroid is so enlarged it is now pressing on and constricting your trachea. You may be experiencing pain, discomfort, or reduced ability to get air in and out, since air moves through the trachea when you breathe. You may also be having difficulty swallowing, since it may also be pressing on your esophagus. The restriction of your trachea and/or esophagus alone may prompt your doctors to want to do surgery to remove your thyroid, since you need a functioning trachea to breathe properly and a functioning esophagus to eat.
As for the thyroid nodules - multinodular goiter is often caused by an increase in TSH (thyroid stimulating hormone) which is produced by the pituitary gland and tells the thyroid to grow. As the thyroid keeps growing, nodules may form. Most thyroid nodules are benign (90-95% are benign), but since the largest nodule is of a significant size, your doctors may want to do a fine needle biopsy on it to determine if it is benign or not (if they aren't already planning to do surgery based on airway restrictions).
"No evidence of suprahyoid or infrahyoid lymphadenopathy by size criteria" - this is looking for swollen lymph nodes, which can be a sign of thyroid cancer spreading outside the thyroid tissue. No evidence of lymphadenopathy is a good sign.
"The bilateral parotid and submandibular glands are symmetric in appearance and without discrete mass lesion or calculus. The parapharyngeal, masticator, and buccal spaces are symmetric in appearance.
The visualized lungs are clear. The orbital globes and retrobulbar structures appear grossly unremarkable. The visualized paranasal sinuses and mastoid air cells are relatively well aerated. The visualized intracranial contents are without acute abnormality. " --- this sounds like everything in this section appears normal.
I hope this helps - I think whether you end up needing thyroid surgery will depend on how much the goiter is impairing your trachea/esophagus function or how your doctors deal with the substernal goiter, how much pain it is causing you, as well as potentially results of a fine needle biopsy on the nodules. Also, if possible I would try to get your doctors to determine what caused the goiter to grow. You may have a thyroid hormone problem like hypothyroidism.
I'm so sorry you have to go through this - I had multinodular goiter that wasn't impairing my trachea but one of my nodules ended up being cancerous, so I had to have thyroid surgery. (This does not mean yours is cancer - most nodules are not cancerous).