Suitability of the Common Goldfish for Assay of Thyrotropic Hormone

The unusual responsiveness of the thyroid tissue of goldfish to small amounts of hypophyseal thyrotropin has provided in these experiments the basis for a sensitive, inexpensive assay procedure. In a series of tests in which 115 two-inch-long fish were used, marked growth of the epithelium of the thyroid follicles was produced by crude and relatively purified preparations from the pituitaries of mammals, birds, amphibia, and fishes. One Parkes-Rowlands guinea pig unit, 0.25 mg of a purified, standardized mammalian pituitary thyrotropic powder was much larger than the minimum dose producing thyroid stimulation in the goldfish. Groups of 5 fish receiving the same treatment were kept in one-gallon jars. The hormone was administered intraperitoneally in 5 successive daily injections of 0.2 cc each day, dissolved or suspended in the desired dilution of Ringer's solution. At autopsy, performed on the sixth day, the lower jaw was removed and trimmed. This piece of tissue, about 2 mm by 3 mm in size, was fixed in Bouin's fluid which contains enough acid to eliminate the need of special decalcifying agents The jaw was cut sagittally in IOjμ. sections. Only about 100 serial-sections, estimated to be approximately median sagittal and parasagittal were saved. The height of the epithelium was measured with a calibrated ocular micrometer and an average of 10 measurements from different follicles was taken. The thyroid epithelial response in this cold-blooded animal was very uniform to any given dosage level. The average control epithelial cell measured 1/μ in height, with extreme variations of 0.5μ from this dimension. In strongly stimulated thyroid tissue, epithelia as high as 12μ were seen. It was found convenient to recognize different grades of stimulation. A 50% increase in average follicle cell height (cells 2μ high) was considered the minimal reaction. Increases of 175% (3.5μ), 300% (5μ), and 500% (9 to 10μ), respectively, were adjudged the arbitrary limits for delineation of the reactions referred to as mild, strong, and very strong.