Would a simple addition of Brazil nuts in the diet be helpful?

Possibly. Selenium can decrease the conversion of T4 to T3, but we don’t know necessarily if selenium has anything to do with that particular scenario. You can test for selenium levels and you can also assess their diet. That’s a good way of assessing whether they’re getting enough selenium. If people are eating ocean fish, they’re generally getting enough selenium. If not, then you might want to assess their diet and consider that.