90% of hypothyroidism is caused by autoimmune thyroid disease (Hashimoto's) but 10% is congenital, due to thyroidectomy or RAI, or is idiopathic (unknown cause).
Your thyroid peroxidase antibodies are negative for Hashimoto's. This doesn't conclusively rule out Hashimoto's because some people have negative TPOab but may be positive for thyroglobulin antibodies which NHS rarely tests. You can order private thyroid antibody tests via thyroiduk.org.uk/tuk/testin...
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