Histological and autoradiographical investigations of renal thyroid tissue were made of several species of bony fishes living in Japan, by comparing it with pharyngeal thyroid tissue, and elucidated the following:
The renal thyroid tissue occurs frequently in the cranial part of the kidney in some of the fresh-water-species (Cyprinus carpio; Carassius auratus, wild and domesticated goldfishes; Misgurnus anguillicaudatus).
The renal thyroid tissue is composed of follicles which are distributed singly or in groups in lymphoid tissue found between the urinary components. A considerable number of the renal follicles are found along the posterior cardinal vein and its branches, which run within the kidney.
Parafollicular cells are not found in the renal thyroid tissue. Colloid of the renal follicles is eosinophilic, periodic acid-Schiff positive and a little vacuolated at the periphery.
Autoradiographs indicate that the renal follicles are capable of accumulating radioiodine I¹³¹ abundantly in the colloid to almost the same degree as found in the colloid of the pharyngeal follicles.