Known triggers for autoimmune thyroid disease include cigarette smoke, stress, low selenium levels, seasonal and food
allergies, sex steroids particularly estrogens, excess dietary iodine, and
"Adequate selenium nutrition supports efficient thyroid hormone synthesis and metabolism and protects the thyroid gland from damage by excessive iodide exposure."
Dietary wise, brazil nuts contain the most selenium. Depending on the soil, anywhere from 50 - 90 mcg of selenium per nut.