I'm not an expert, but I recently read Thin For Good by Fred Pescatore, former medical director for the Atkins Center. He recommended kelp (which you already know about) and magnesium for hypothyroidism.

Protein Power has this sentence buried on page 122 in discussion of rampant chromium deficiency (90% of the country is reported to be chromium-deficient):

"Sufficient chromium will help you build lean muscle pounds, and through interaction with the thyroid system, help you burn fat more efficiently."

That doesn't quite say "take chromium to improve thyroid function" but seems to suggest that chromium is required by the thyroid to burn fat.

Hope that's of some help.

Allison