BACKGROUND Congenital thyroid anomalies can be caused by abnormal descent or agenesis of part of the thyroid gland. Agenesis of the thyroid gland may involve one or both lobes of the gland, with or without isthmus, and is very rare. Congenital thyroid anomalies have been reported to be associated with thyroid diseases (hyperthyroidism, adenomatous goiter, and cancer) but not with parathyroid diseases. METHODS Case reports of two patients describe the methods used. RESULTS We report two patients with hyperparathyroidism and congenital thyroid anomalies. One patient had a left thyroid lobe hemiagenesis and the other patient had agenesis of the thyroid isthmus. Both patients had primary hyperparathyroidism caused by parathyroid hyperplasia and were treated by subtotal parathyroidectomy. The parathyroid glands and recurrent laryngeal nerves were in their usual positions despite the abnormal development of the thyroid glands. CONCLUSIONS When thyroid hemiagenesis or isthmus agenesis is found in patients undergoing parathyroidectomy for hyperparathyroidism, parathyroid hyperplasia should be suspected. If parathyroid hyperplasia is present, subtotal parathyroidectomy and bilateral thymectomy are recommended. The parathyroid glands and the recurrent laryngeal nerves can be found in their usual locations.