Hashimoto's thyroiditis is an autoimmune condition in which the immune system slowly damages the thyroid. It is the most common cause of hypothyroidism and is much more common in women, often appearing in midlife.
It's diagnosed with thyroid blood tests including TSH and thyroid antibodies, and managed by replacing the missing hormone with levothyroxine. There is no diet that cures it — see Hashimoto's disease for what the evidence does and doesn't support.