Element essential to thyroid function

I believe the answer is:

iodine

(Other definitions for iodine that I've seen before include "Mother-of-pearl" , "Tincture for cuts" , "Nonmetallic element in seaweed" , "Element forming black crystals or violet vapour" , "Element that can exist as a purple vapour" .)

I've seen this clue in The New York Times.

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