Is this dosage dangerous?

I believe Synthroid (levothyroxine) is only available in microgram doses not milligrams. 0.5mg would be 500mcg which would be a more than an extremely high size dose. I only take 50mcg.
I'd double check with your doctor and/or pharmacist to be sure of what you're taking.

First be absolutely sure the dosage reads 5mg instead of 5mcg. Huge difference. The 5mcg dose is quite normal and safe. If it really does read 5mg, that would be quite strong. To be honest I wouldn't think synthroid even came in a 5mg dose. If it says 5mg I would take it back to the pharmacy and question whether there may be a misprint on the label, or that no one misread the dr's prescription. If those things have been verified, you can always talk to your pharmacist and ask for their opinion. They should be able to either reassure you about the dosage or suggest you contact your Dr on Monday and double- check with him.