The thyroid is a major endocrine gland present in craniate vertebrates that functions chiefly in metabolism and growth. In humans, the two connected lobes of follicular tissue that comprise the gland are located on either side of the trachea, but the gland is positioned differently in various other species. The thyroid gland secretes thyroxine, a hormone that regulates the oxidation rate of cells and contains a large amount of iodine. Insufficient iodine in the diet can interfere with the production of thyroxine, resulting in hypothyroidism, which is distinguished by swelling of the thyroid and neck known as goiter. Hypothyroidism of this type has been virtually eliminated by the widespread use of iodized salt. The condition may also develop, however, due to problems with glandular function, in which case it is most commonly referred to as either cretinism (childhood form) or myxedema (adult form). Studies of the rat thyroid have been critical in developing a better understanding of these and other thyroid diseases.

Thyroid follicles are composed of a stratum of epithelial cells and contain a colloid rich in thyroglobulin. The protein plays an important role in the production of thyroxine. Blood tests are available that can measure the level of thyroglobulin in the body, and these tests are particularly useful for certain cancer patients. Individuals with active thyroid cancer may exhibit unusually high levels of the protein. Following successful surgical removal of the thyroid, thyroglobulin should no longer be detectable. Consequently, thyroglobulin testing can reveal whether or not all thyroid cancer cells have been eliminated.

The section of rat thyroid tissue featured in the digital image above was labeled with Oregon Green 488 conjugated to wheat germ agglutinin, a lectin that selectively binds to sialic acid residues found in both mucoproteins and glycoproteins. The cells were also stained with Alexa Fluor 568 conjugated to phalloidin and the dye Hoechst 33342, which target the cytoskeletal filamentous actin network and nuclear DNA, respectively. Images were recorded in grayscale with a 12-bit digital camera coupled to a Nikon Eclipse 80i microscope equipped with bandpass emission fluorescence filter optical blocks. During the processing stage, individual image channels were pseudocolored with RGB values corresponding to each of the fluorophore emission spectral profiles.