The thyroid gland is composed of two lobes that are joined together by a central mass called the isthmus. The gland is inferior to the larynx. The thyroid gland produces these hormones:

(1) "thyroid_metabolic hormones" and

(2) calcitonin, which is also known as thyrocalcitonin.

The "thyroid metabolic hormones" are two separate substances (1) T₃, known as triiodothyronine and (2) T₄, known as thyroxine or tetraiodothyronine. Both T₃ and T₄ help regulate metabolism. Calcitonin decreases blood calcium levels by stimulating calcium deposits in bones.

The thyroid is composed of structures called follicles, which contain colloid. Colloid contains thyroglobin. This substance the precursor for T₃ and T₄ . The cells that make up the follicle border are called follicular cells. Scattered among, but external to, the follicles are parafollicular (C-cells) cells. These cells produce calcitonin.

Parathyroid glands are located on the posterior side of the thyroid gland. There are usually two oval parathyroids on each lobe of the thyroid, but sometimes there are more.

The chief_cells are the most numerous cells in the parathyroid. They produce parathyroid_hormone or PTH, which regulates the removal of calcium from bones to regain calcium homeostasis in the bloodstream. Human parathyroids (but not the parathyroids of other mammals) have additional cells, which are called oxyphil_cells. The function of the oxyphil cells is still unknown.

On this slide, the chief cells are plentiful and have a dark-staining nucleus with a lighter stain in their cytoplasm. The oxyphil cells (few in number) have a dark-staining nucleus, but clear cytoplasm.