The thyroid gland of normal specimens of the teleost, Seriola quinqueradiata, about 50–70 cm in length, obtained in April, were studied with the electron microscope. The apical surface of the follicular cell is irregular in outline and has fewer microvilli than that in higher vertebrates. The rough-surfaced endoplasmic reticulum is well developed throughout the cytoplasm of the follicular cell, as in the higher vertebrates, and free ribosomes are widely distributed in the matrix of the cytoplasm.

In all the follicular cells, there are several less dense or moderately dense droplets which are considered to be derived from Golgi elements. In some droplets occur aggregates of numerous wavy, fine filaments and crystal consisting of groups of thick needle-like fibrils. Each fibril is 110–120 Å in diameter and composed of three layers comprising a less dense layer 35–40 Å thick, between two layers 35–40 Å in thickness. Dense granules such as those commonly found in the higher vertebrates were not observed in the thyroid cell of this fish, though a few round or oval bodies, which might be lysosomes, containing small vesicles, membranous structures, or wrapped whorled lamellae were noticed.

The fine structure of the pericapillary region is similar to that of the higher vertebrates, though the endothelial pores are not observed.