The important thing in a patient in whom platelet values were detected to be greater than normal is whether thrombocytosis is a reactive phenomenon due to a different pathology or due to a clonal hematological pathology. In this case report, reactive thrombocytosis observed in a case with subclinical hypothyroidism due to Hashimoto's thyroiditis is reported; and according to our literature review, this is the first reported case of reactive thrombocytosis due to Hashimoto's thyroiditis and/or subclinical hypothyroidism. A 31-year-old man without any complaint was admitted to the Hematology Department for thrombocytosis which was detected in his routine follow-up. He had been using thyroid hormone replacement for 2 years because of hypothyroidism as Hashimoto's thyroiditis was earlier diagnosed. Due to miscommunication, he stopped his medication levothyroxine 0.1 mg/day 2 weeks in advance and he was still off-drug on admission. Platelet count was 715 × 109/l in the first admission center. Subclinical hypothyroidism diagnosis was made with the present findings and thyroid hormone replacement therapy was again commenced gradually. Platelet counts and thyroid-stimulating hormone value were normal 6 weeks later. As a result, before making diagnosis of thrombocytosis related to myeloproliferative disease or myelodysplastic syndromes, secondary causes should be investigated carefully.