Berries are a powerful superfood due to their antioxidant powers, Monica Moore, owner and registered dietitian nutritionist at Body by Monica, says. Berries are also very high in fiber – about 6 grams per serving. They are also a great option if you want to keep your thyroid healthy. Their low Glycemic Index means no insulin spikes and less fat storage. Research has shown that people with underactive thyroid have higher levels of harmful free radicals.