The parathyroid glands are four tiny glands in your neck that control how much calcium circulates in your blood. Each one is roughly the size of a grain of rice, and despite their name, they have nothing to do with thyroid function. Their sole job is producing parathyroid hormone (PTH), which keeps blood calcium within a narrow range of about 8.6 to 10.3 mg/dL, a level critical for your muscles, nerves, and bones to work properly.
Location and Physical Structure
The four parathyroid glands sit on the back surface of the thyroid gland, two on each side. They’re easy to miss: each weighs only a few milligrams. Most people have exactly four, but anatomy studies show about 5% of people carry a fifth gland, while roughly 2% have only three. The upper pair tends to be slightly larger than the lower pair.
In about 8.5% of cases, one or more glands end up in an unusual location. They can be embedded within the thyroid tissue itself, positioned elsewhere in the neck, or even located in the chest cavity behind the breastbone. These variations are typically harmless but can complicate surgery if a gland becomes diseased and doctors need to find it.
How Parathyroid Hormone Controls Calcium
When calcium in your blood dips even slightly, the parathyroid glands release PTH. This hormone acts on three parts of the body simultaneously to bring calcium back up.
In your bones, PTH triggers cells called osteoclasts to break down small amounts of bone tissue and release stored calcium into the bloodstream. This process is tightly regulated. Continuous, unrelenting PTH release causes bone loss over time, but brief, intermittent pulses of PTH can actually stimulate bone building. This dual nature is the reason a synthetic form of PTH is used to treat osteoporosis.
In your kidneys, PTH does two things at once. It tells the kidney to hold onto calcium that would otherwise leave in your urine, and it blocks the reabsorption of phosphate, flushing it out instead. This matters because calcium and phosphate levels work like a seesaw: lowering phosphate helps raise the effective concentration of usable calcium.
PTH also activates vitamin D in the kidneys. The vitamin D you get from sunlight or food starts in an inactive form. PTH triggers a specific enzyme in kidney cells that converts it into its fully active form (calcitriol). That active vitamin D then travels to your intestines and increases how much calcium you absorb from food. So even though the parathyroid glands don’t interact with your gut directly, they control calcium absorption there by flipping the vitamin D switch in the kidneys.
The Difference Between Parathyroid and Thyroid
People often confuse these because of the shared name and location, but the two serve completely different purposes. The thyroid gland produces hormones that regulate metabolism, heart rate, and body temperature. The parathyroid glands focus exclusively on calcium and phosphate balance. You can have a perfectly healthy thyroid and a diseased parathyroid, or vice versa. The only real connection is physical: they’re neighbors, and surgery on one can accidentally affect the other.
Overactive Parathyroid Glands
Primary hyperparathyroidism occurs when one or more parathyroid glands produce too much PTH regardless of how much calcium is already in the blood. It affects roughly 1% of adults, with women developing it about twice as often as men. The most common cause is a benign growth (adenoma) on a single gland. Less frequently, two or more glands enlarge together.
The classic symptoms are sometimes summarized as “stones, bones, groans, and moans.” Kidney stones form because excess calcium spills into the urine. Bones weaken and fracture more easily as PTH continuously pulls calcium from them, sometimes progressing to osteoporosis. Many people experience abdominal discomfort, fatigue, depression, and difficulty concentrating. Frequent urination and increased thirst are also common.
In practice, though, many cases are caught before symptoms appear. A routine blood test reveals high calcium, which prompts further testing. A normal PTH level falls between 11 and 51 pg/mL. In primary hyperparathyroidism, PTH is elevated (or inappropriately “normal”) alongside elevated calcium, a combination that signals the feedback loop is broken.
Secondary hyperparathyroidism is a different situation. Here the parathyroid glands are responding correctly to genuinely low calcium, usually caused by chronic kidney disease or severe vitamin D deficiency. Because the kidneys can’t activate vitamin D properly, calcium absorption drops, and the parathyroids work overtime to compensate. Treatment focuses on the underlying cause rather than the glands themselves.
When Surgery Is Recommended
For primary hyperparathyroidism, surgical removal of the overactive gland (parathyroidectomy) is the definitive treatment. It’s recommended for anyone with symptoms like kidney stones, osteoporosis, or fractures. Even without symptoms, surgery is typically advised for people under 50, those with calcium levels more than 1 mg/dL above normal, or anyone with significantly reduced bone density or kidney function. The procedure has a high success rate and most people go home the same day or the next morning.
Underactive Parathyroid Glands
Hypoparathyroidism, where the glands produce too little PTH, is less common but can be serious. The most frequent cause is accidental damage during thyroid or neck surgery. Because the parathyroid glands are so small and sit directly on the thyroid, they can be bruised, cut off from their blood supply, or inadvertently removed. Autoimmune conditions and rare genetic disorders account for most remaining cases.
Without enough PTH, blood calcium drops. The symptoms stem directly from that deficit. Low calcium makes nerves and muscles overly excitable, leading to tingling or burning in the fingertips, toes, and lips. Muscle cramps can affect the legs, feet, abdomen, and face. In more severe cases, the hands lock into painful, sustained spasms, and the muscles around the throat can tighten enough to interfere with breathing.
Beyond muscle symptoms, people with hypoparathyroidism often report fatigue, weakness, headaches, and memory difficulties. Treatment typically involves calcium and active vitamin D supplements to replace what the missing hormone would normally provide. Most people manage well on this regimen, though it requires regular blood work to keep calcium levels stable.
How Calcium Balance Affects Daily Life
Calcium isn’t just about bones. Your body uses it to transmit nerve signals, contract muscles (including your heart), and clot blood. The parathyroid glands keep blood calcium within that tight 8.6 to 10.3 mg/dL window because even small deviations cause noticeable problems. Too high, and you feel sluggish, confused, and dehydrated. Too low, and your muscles twitch and cramp involuntarily.
Vitamin D plays a supporting role in this system, and the parathyroid glands are the link between the two. If you’re chronically low on vitamin D, your parathyroid glands have to work harder to maintain calcium levels, potentially enlarging over time. This is one reason vitamin D deficiency is taken seriously even in people who feel fine: the parathyroid glands may be silently compensating, pulling calcium from bones to keep blood levels normal.
For most people, the parathyroid glands work in the background without any issues. Problems tend to surface either through routine blood tests showing abnormal calcium, or through symptoms like unexplained kidney stones, bone fractures, or persistent muscle cramps that prompt further investigation.