How should I monitor thyroid function in a patient diagnosed with Hashimoto's Thyroiditis who is on levothyroxine therapy?

For a patient diagnosed with Hashimoto's thyroiditis receiving levothyroxine therapy, monitor thyroid function by measuring serum TSH levels every 3 months until the TSH stabilises within the reference range on two occasions 3 months apart, then measure TSH once yearly thereafter.

If symptoms of hypothyroidism persist despite TSH being within the reference range, consider measuring free thyroxine (FT4) to guide further dose adjustment.

Aim to maintain TSH within the reference range to avoid under- or overtreatment, and be aware that TSH may take up to 6 months to normalise after starting or adjusting levothyroxine, especially if initial TSH was very high or hypothyroidism was longstanding.

Do not routinely repeat thyroid peroxidase antibody (TPOAb) testing once diagnosis is established.