My mom and I both have chronic sinusitis.
First of all, you need to be retested for the thyroid problem. This can be very serious, and you need to have it taken care of. My mom had to have her thyroid removed - but medicine has come a long way since then.
Sometimes with chronic sinusitis a "rhinoscopy" is appropriate. Only an ear, nose, and throat doctor can make that determination. Both my mom and Gary have had to have one - it is a fairly simple outpatient procedure that takes just a morning or afternoon (cleaning and medicating the sinuses surgically) and full recovery is a few days. HOWEVER, my mom lost her sense of smell (and taste) for over a year after the procedure. Gary lost his for a few days.
To avoid ever having to have a rhinoscopy, the very best thing you can do is a daily sinus rinse. It may feel uncomfortable at first, but it is something Indians practicing ayurvedic "medicine" have used for thousands of years. Any good allergist or ENT doctor recommends them as well. Here's a link to the product I use: http://www.neilmed.com/
You can make your own mixture of saline solution and purchase a "neti" pot: http://store.bytheplanet.us/neti-pots-neti-pots.html
. But make sure you use DISTILLED water and not spring water or tap water, as these can lead to infection. And if you're going to make your own saline solution, use kosher salt, as it has been purified.
With chronic sinusitis, it is important to keep the inflammation down. The sinus rinse itself may be enough, though it will likely take a few days for you to see the improvement. If the sinus rinse itself doesn't do it, you may want to talk to a Doctor about a steroid nasal spray, like Nasonex, Flonase or Nasocort. These are nasal sprays that treat the area directly, and are far, far better for your health in the long run than continuing to take decongestants. The nasal steroids are used twice a day. And if you do the nasal rinse at night, you would take the night time nasal steroids AFTER the nasal rinse.
Almost everyone I know has their sinus infections treated with Levaquin when using a drug. However, many allergists and ENT doctors around here have stopped prescribing anything for them because the problem has become so rampant that all we're doing is creating multi-strain resistant bacteria. Many allergists around here simply recommend the daily nasal rinse (sometimes called nasal irrigation) in combination with the steroid spray (which does require a prescription).
It is also a possibility that your congestion is a reaction to allergies. That would explain why the antibiotics haven't been working. Have you been to an allergist? Just another avenue to explore.