First blood result hyperthyroid, second time better but not perfect. ultrasound was clear not enlarged, no nodule. what could cause this fluctuation?

Subacute thyroiditis: Hi. Fluctuations from normal to hyperthyroid falling back through normal to hypothyroid, then usually returning to normal is characteristic of a subacute thyroiditis. The hyper phase is self-limited and is due to leakage of stored hormone, not overproduction (so very different than Graves'). When the stored hormone is depleted, there is a hypo phase until the thyroid recovers and is normal again