How could past chemo and radiotherapy relate 20 years later with very high tsh level production?

You are hypothyroid.: Which may or may not be related to the previous therapy. Was the radiation to your neck?

Probably no relation: There may not be any relationship. Elevated TSH is seen with hypothyroidism which can occur for many reasons including autoimmune disorders. It is possible that radiation to the neck or chemotherapy may cause damage to the thyroid cells but this is unlikely although not impossible. The duration of 20 years would suggest this is not the cause. Look elsewhere for the cause of high tsh.