Thyroid nodule smear showing hematoidin crystals

This image shows a smear of a thyroid nodule where the cells are undergoing cell death due to restricted blood supply. Irregularly-shaped necrotic (dead) thyroid follicular cells can be seen on the left side of the image (blue). Lymphocytes, a type of white blood cell, are also present (small, dark blue cells). Thyroid follicular cells use iodine from the blood supply for the production of thyroid hormones. The yellow 'splashes' in this image are hematoidin, a pigment derived from hemoglobin, the oxygen carrying protein found in red blood cells (erythrocytes). Hematoidin forms when erythrocytes degenerate under conditions of low oxygen tension in a closed tissue compartment. Porphyrin is released from hemoglobin and converted to biliverdin which is in turn reduced to crystalline hematoidin (yellow). The hematoidin crystals are sometimes described as resembling cockleburs. Horizontal image width 215 micrometers.