A 9-year-old girl is brought to the office by her mother who has noticed that the child’s neck looks “odd.” The girl states that it feels “like something is there,” indicating the base of the neck, but that she has no difficulty swallowing. A thorough examination reveals only a firm, nontender, and diffusely enlarged thyroid with no thrills or bruits. All parameters on a thyroid function panel are within normal limits. Low levels of thyroid antibodies are present. What is this child’s most likely diagnosis?