Agree with the above, but more details would be useful. Did anything happen that might have changed her attitude to strangers? Is it possible someone inadvertently hurt her? Is it all strangers now, and is she O.K. with people she has met previously, or is it an issue with any visitor? You mention her "jumping up to say hello". Apart from anything else, it is bad doggy manners to jump on people, and you might want to teach her to greet with four feet on the ground. A stranger reaching for a dog is IMO always a bad idea. It can be perceived as aggression. Better to let the dog do her sniffing and investigation, then offer the back of a hand not to pet but for the dog to sniff and accept, before any attempt to touch. Does she growl in warning and give the stranger a chance to react and back off, or is the bite immediately delivered as she growls?
Oh, and one other thought: does she have any sore areas on her body? Any chance that the act of jumping up hurts her and she associates that with the visitor? (does she jump on you with no adverse reaction?) If the thyroid turns out not to be an issue, then one wonders what has changed. Not coming into heat could be a sign of something going on…...I had a situation with a mare who had cramps when she ovulated, and was "off" to the point of looking like colic, so "female troubles" can show up in unusual ways. It took us a while to diagnose her difficulties.