['To determine the frequency of nutritional deficiencies and thyroid dysfunction in children with celiac disease (CD) and during follow-up after initiation of a gluten-free diet. Laboratory investigations of hemoglobin, ferritin, calcium, folate, vitamin B12, vitamin D, and thyroid function are regularly ordered in children with CD despite sufficient evidence for these.', 'Between 2009 and 2014, test results of hemoglobin, ferritin, folate, vitamin B12, calcium, vitamin D (25[OH]D), free thyroxin, and thyroid stimulating hormone of children with CD regularly seen at the Leiden University Medical Center were investigated. Laboratory reference ranges were used to define abnormal results. Pearson χ(2) test for trend, unpaired t test, and 1-way ANOVA were used for statistical analysis.', 'Of the 182 children evaluated, 119 were newly diagnosed. On average, 17% of results per year were missing because of incomplete blood investigations. Iron deficiency (28%) and iron deficiency anemia (9%) were found at the time of diagnosis of CD. Folate (14%), vitamin B12 (1%), and vitamin D deficiencies (27%) were also seen. No hypocalcemia or thyroid dysfunction was found. At follow-up, iron deficiency, iron deficiency anemia, and folate and vitamin D deficiency were observed in 8%, 2%, 3%, and 25% of patients, respectively. Vitamin B12 deficiency, hypocalcemia, and thyroid disease were not found.'] Does complementary Serologic Investigations in Children with Celiac Disease be Unnecessary during Follow-Up?