Thats a good question since I asked my dr. and she told me it was the same thing? What is the difference?
Hashimoto’s thyroiditis is the most common cause of hypothyroidism in the western world. Infiltration of lymphocytes into thyroid tissue can lead to thyrocyte destruction and
ultimately hypothyroidism. In short, Hashimoto's thyroiditis comes first.
Selenium is essential for the conversion of T4 to T3 and also plays a role in protecting the thyroid gland itself. Studies show selenium decreases thyroid peroxidase antibodies (TPOAb). The best source of selenium in the diet are Brazil nuts.
Hashimoto's causes hypothyroidism when antibodies destroy thyroid tissue; however, many of us aren't diagnosed with Hashimoto's until "after" we've been diagnosed with hypothyroidism because that's when most of the tests are actually done. Some doctors actually refuse to do antibody testing to diagnose Hashimoto's (my doctor did) because Hashimoto's is the # 1 cause of hypothyroidism in developed countries so they simply assume that's the cause. Some simply call it an "idiopathic" issue, meaning there's no cause, better taken to mean they don't care to look for a cause.
Sometimes, people are diagnosed with hypothyroidism but don't have antibodies so the cause isn't readily determined. Hashimoto's can be diagnosed via thyroid ultrasound determined by certain characteristics consistent with Hashimoto's.
In addition, not all hypothyroidism is caused by Hashimoto's. Sometimes it's caused by Central, otherwise known as Secondary, hypothyroidism, which is a pituitary or hypothalamus issue not an autoimmune condition. Other times, it's a sub-clinical problem that resolves itself over time or often turns into an autoimmune condition.
Because of the various possibilities, it would easier for us, if you'd post your thyroid-related blood test results, including Free T4, Free T3, TSH, any antibody testing, etc. Be sure to include all reference ranges for any results since ranges vary from lab to lab and have to come from your own reports for the best comparison.
Not everyone needs to be on a special diet, although some of us do better if we eliminate certain foods that can cause problems. That can be easily determined with an elimination diet; however, eliminating foods you don't need to eliminate can cause deficiencies that can make matters worse.