A 9 1/2 -year-old girl presents to your office with an approximately 1 1/4 -cm nodule in her neck, just to the left of the midline and below her cricoid cartilage. It is nontender and moves when she swallows. It has been enlarging over the last several months and was not seen by the pediatrician at her 9-year-old check-up. There is no family history of endocrine disorders. The most likely diagnosis is

While reactive lymphadenopathy is by far the most common cause of neck masses in children; a lymph node should not move with deglutition and is more likely to be tender. A branchial cleft syst should be more lateral and the thyroglossal duct cyst should be higher and in the midline (although they can sometimes present off the midline). One might also have expected some prior evidence of both of these congenital cysts, although that is not always the case. The location and characteristics strongly suggest a thyroid nodule. While follicular adenomas are much more common in adults than cancers they are rarer in children and a rapidly growing solitary nodule is likely to be a papillary carcinoma, the most common thyroid cancer in children.