Complex sentence: Subacute lymphocytic thyroiditis features a small goiter without tenderness and tends to have a phase of hyperthyroidism followed by a phase of hypothyroidism, and then a return to a euthyroid state.
Split into simpler sentences: Subacute lymphocytic thyroiditis features a small goiter without tenderness. This condition tends to have a phase of hyperthyroidism followed by a return to a euthyroid state, and then a phase of hypothyroidism, followed again by a return to the euthyroid state.