Parathyroid hormone (PTH) is the body’s primary regulator of calcium in the blood. It works around the clock to keep your blood calcium within a very tight range, pulling from your bones, adjusting what your kidneys filter, and activating vitamin D so your gut can absorb more calcium from food. Normal PTH levels in adults fall roughly between 15 and 65 pg/mL.
How PTH Keeps Calcium in Balance
Your body treats blood calcium like a thermostat setting. When calcium drops even slightly, four tiny glands in your neck, each about the size of a grain of rice, detect the change and release PTH into the bloodstream. PTH then acts on three targets simultaneously: bones, kidneys, and the intestines (indirectly through vitamin D). The combined effect pushes calcium back up to its normal range.
When calcium rises high enough, a sensor on the surface of parathyroid cells called the calcium-sensing receptor shuts down the response. This receptor detects the higher calcium and reduces PTH production, PTH secretion, and even the growth of parathyroid cells. It’s a continuous feedback loop, adjusting PTH output minute by minute throughout the day.
What PTH Does to Your Bones
Bone is not a static structure. It’s constantly being broken down and rebuilt. PTH tips this balance toward breakdown when your body needs calcium. It signals bone-embedded cells called osteocytes to produce a chemical messenger that activates osteoclasts, the cells responsible for dissolving bone tissue and releasing stored calcium into the blood. At the same time, PTH slows down the activity of osteoblasts, the cells that build new bone.
This sounds alarming, but in short bursts it’s completely normal. Your skeleton is a massive calcium reservoir, and small, temporary withdrawals don’t weaken it. Problems only arise when PTH stays elevated for long periods, which gradually thins bone and increases fracture risk. Interestingly, when PTH is given in small, intermittent doses as a medication, it actually stimulates bone formation rather than breakdown, which is why synthetic PTH is used to treat severe osteoporosis.
How PTH Works in the Kidneys
Your kidneys filter about 180 liters of fluid per day, and a lot of calcium passes through that filter. PTH tells the kidneys to reclaim calcium rather than let it leave in your urine. It does this at multiple points along the kidney’s filtration tubes. In one segment, PTH increases the permeability of the tissue so calcium passes back into the blood more easily. In another, it boosts the number of calcium channels on cell surfaces, pulling even more calcium back in.
At the same time, PTH has the opposite effect on phosphate. It blocks phosphate from being reabsorbed, so more phosphate gets excreted in urine. This matters because calcium and phosphate bind to each other in the blood. If both were high at the same time, they could form deposits in soft tissues. By dumping phosphate while saving calcium, PTH keeps the two minerals in a safe ratio.
PTH Activates Vitamin D
The vitamin D you get from sunlight or supplements is not in its active form. It has to be converted by the liver first, and then by the kidneys, before it can do anything useful. PTH controls that final kidney step. It increases the production of the enzyme that converts vitamin D into its active form, calcitriol. Calcitriol then travels to the intestines and ramps up calcium absorption from the food you eat.
This is why vitamin D deficiency and PTH problems are so closely linked. If you don’t have enough vitamin D for PTH to activate, your intestines can’t absorb calcium efficiently, and PTH has to work harder, pulling more from bone and reclaiming more from the kidneys. Over time, this can lead to chronically elevated PTH even when the parathyroid glands themselves are perfectly healthy.
Magnesium’s Hidden Role
Magnesium doesn’t get as much attention as calcium or vitamin D, but the parathyroid glands need it to function. Magnesium is required for both the production and the release of PTH. When magnesium drops too low, the glands can’t secrete enough PTH, and blood calcium falls as a consequence. This is called secondary hypocalcemia, meaning the low calcium is a downstream effect of the magnesium problem. Correcting the calcium alone won’t fix it. The magnesium deficiency has to be addressed first.
What Happens When PTH Is Too High
Primary hyperparathyroidism occurs when one or more parathyroid glands overproduce PTH, usually because of a benign tumor on the gland. The excess PTH drives blood calcium above normal levels. Many people with this condition have no obvious symptoms, and it’s often caught incidentally on a routine blood test showing elevated calcium.
When symptoms do appear, they tend to be vague: fatigue, difficulty concentrating, depression, and a general sense of not feeling well. Over time, persistently high calcium can cause kidney stones, bone thinning, frequent urination, and digestive problems. The classic medical shorthand is “stones, bones, groans, and moans,” referring to kidney stones, bone pain, abdominal complaints, and mood changes.
What Happens When PTH Is Too Low
Hypoparathyroidism, where PTH production drops below what the body needs, leads to low blood calcium. This makes nerves and muscles overly excitable. Early signs include tingling around the mouth, numbness in the fingertips, and muscle cramps. More severe deficiency can cause painful spasms in the hands and feet, where the fingers and wrist lock into a characteristic flexed position.
The most common cause is accidental damage to the parathyroid glands during thyroid surgery, since the glands sit right behind the thyroid. Autoimmune destruction and genetic conditions are less common causes. Treatment focuses on calcium and active vitamin D supplementation to replace what PTH would normally regulate.
Biotin Supplements Can Skew PTH Tests
If you take biotin supplements for hair, skin, or nail health, be aware that biotin can interfere with the lab test used to measure PTH. The unbound biotin molecules in your blood mimic a component used in the test, which can produce falsely low PTH readings. In documented cases, patients taking 1,500 to 5,000 micrograms of biotin per day had PTH results that looked abnormally low, leading to unnecessary workups or missed diagnoses of hyperparathyroidism. Stopping biotin for a few days before blood work allows the test to return accurate results.