They distinguish themselves from the thyroid gland histologically as they contain two types of cells:
|parathyroid chief cells||darker||many||smaller||manufacture PTH (see below).|
|oxyphil cells||lighter||few||larger||function unknown.|
Parathyroid hormone (PTH, also known as parathormone) is a small protein that takes part in the control of calcium and phosphate homeostasis, as well as bone physiology. Parathyroid hormone has effects antagonistic to those of calcitonin. It increases blood calcium levels by stimulating osteoclasts to break down bone and release calcium. It also increases gastrointestinal calcium absorption by activating vitamin D, and promotes calcium uptake by the kidneys.
Modern high frequency ultrasound can see parathyroid masses, even before they cause high calcium. They are called parathyroid incidentalomas. If a patient has elevated calcium, the ultrasound can be used to locate the abnormal glands. The use of ultrasound guided FNA, and parathyroid hormone washings can confirm the abnormal glands. A blood calcium 15-30 minutes after the biopsy can help determine if the disease is caused by a single abnormal gland or multiple glands.
A drop in serum calcium suggests a single source, and no drop suggests multiple glands. This, with a non-localizing Sestamibi scan would point toward a neck exploration, rather than a minimally invasive method aimed a single gland disease.
A Sestamibi scan is often used to determine which parathyroid gland(s) are responsible for overproduction of parathyroid hormone.
Another related condition is called secondary hyperparathyroidism, or secondary HPT - common in patients with chronic kidney disease on dialysis. In secondary HPT, the parathyroid glands make too much parathyroid hormone (PTH), and the kidneys do not produce enough vitamin D, and calcium and phosphorus are out of balance. Even though one may not have any symptoms, treating secondary HPT is important. Cinacalcet (Sensipar) is a medicine that can help treat such dialysis patients and is available by prescription only.
Genetically, Eya-1 (transcripitonal co-activator), Six-1 (a homeobox transcription factor), and Gcm-2 (a transcription factor) have been associated with the development of the parathyroid gland, and alterations in these genes alters parathyroid gland development.
The conserved homology of genes and calcium sensing receptors in fish gills with those in the parathryroid glands of birds and mammals is recognized by evolutionary developmental biology as evolution using genes and gene networks in novel ways to generate new structures with some similar functions and novel functions.