Experimental hyperthyroidism in urodele larvæ (Amblystoma) and anuran larvæ (Rana, Bufo, and Hyla) is accompanied by definite changes in bile color. The normal pale green, or pale yellow-green, color of the full gall bladder changes progressively after thyroid administration to a brighter green, then emerald-green, and finally a very dark green. In several hundred observations no exceptions were noted.
The bile pigment, biliverdin (and its derivatives), is elaborated from the hemoglobin of worn out erythrocytes. Thyroid administration induces an increased rate of erythrocyte destruction, and this is followed by an increased output of bile pigment. Other minor factors are mentioned which may to a limited extent modify the color of the bile.
Erythrocyte destruction occurs largely by enucleation, cytoplasmic segmentation, and fragmentation, and is probably widespread in the body. Many fragments and senile red cells collect in the liver. During the later stages of thyroid treatment the macrophages become conspicuously active. They are especially abundant in the liver, the gut, and the gills. In addition to the hemoglobin eliminated after transformation into bile pigment, some is transported by macrophages through the gut lining, and to a less extent through the involuting gill epithelium, and thus eliminated from the body.