I usually leave my lapel pin in my suit jacket, but if necessary I can pack it into my check-in luggage.
For a domestic flight within the U.S., is this something I need to worry about?
Yes, you can wear it.
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You may transport this item in carry-on or checked baggage.
I do not believe there is any need to pack it into check-in luggage. Just like any other metal items on you, for example a belt buckle, you can take the pin off the jacket and put it in the tray with your other items while you cross the security screening gate.
What can I do to prevent an alarm?
To reduce the likelihood of an alarm that results in pat-down screening, remember to remove all items from your pockets before you go through screening. You should avoid wearing clothes, shoes and jewelry with a high metal content.
Specially that the lapel pin is not a prohibited item as declared by TSA. Safety Pins, Bobby Pins and Stick Pins are all allowed both in checked bags and carry on bags. I'm sure a clothing item allowed in carry on bag is also allowed to be worn, even though its best to put it on the belt to avoid triggering the alarm.