The thyroid gland is a component of the endocrine system comprised of two oblong lobes linked by a narrow piece of tissue called the isthmus. The thyroid is located in the throat and contains millions of follicles that act as storage sites for the various hormones produced by the gland.

Thyroid secretions function in a variety of ways in the human body but are particularly important in adults as regulators of metabolism. The hormones are also central to normal growth and development in children. A childhood deficiency in thyroid hormones has been linked to dwarfism and mental retardation.

Hormones are stockpiled in follicles of the thyroid gland in the form of the glycoprotein thyroglobulin. Prior to secretion into the bloodstream, however, most of the hormones are converted into thyroxine, though small amounts of related substances are also manufactured. The amount of thyroid hormones that are generated primarily depends upon the level of thyroid-stimulating hormone released by the pituitary gland. An efficient feedback mechanism ensures that the bodily level of thyroxine generally remains within a steady range. Nevertheless, certain medical conditions or an insufficient supply of iodine can cause the thyroid to produce abnormal quantities of hormones, resulting in a variety of disorders, the best known of which is goiter.

The human thyroid gland tissue sample featured in the digital image above was fluorescently labeled with Alexa Fluor 488 conjugated to phalloidin, targeting polymerized actin (F-actin). The specimen was also stained with Texas Red-X conjugated to wheat germ agglutinin, which selectively binds to N-acetylglucosamine and N-acetylneuraminic (sialic acid) residues and is popular for staining the Golgi network in fixed cells and tissues. DNA was targeted with 4',6-diamidino-2-phenylindole (DAPI). Images were recorded in grayscale with a 12-bit digital camera coupled to either a Nikon E-600 or 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.