Calcitonin is a hormone that lowers calcium levels in your blood. It does this through two main pathways: slowing the breakdown of bone and reducing the amount of calcium your kidneys send back into your bloodstream. Your body uses calcitonin as a counterbalance to parathyroid hormone, which raises blood calcium. Together, these two hormones keep calcium in a tight, healthy range.
Where Calcitonin Comes From
Calcitonin is produced by specialized cells in your thyroid gland called parafollicular cells, or C cells. These cells have calcium-sensing receptors on their surface, which means they can detect when calcium in the blood rises above normal. When they sense that spike, they release calcitonin into the bloodstream to bring levels back down.
This makes the system essentially self-regulating. High blood calcium triggers calcitonin release, calcitonin lowers blood calcium, and the signal to produce more calcitonin fades. It’s a straightforward feedback loop.
How It Lowers Blood Calcium
Calcitonin works on two main targets: your bones and your kidneys.
In bone, it acts on osteoclasts, the cells responsible for breaking down bone tissue. Normally, when osteoclasts dissolve bone, the calcium stored in that bone enters your bloodstream. Calcitonin binds to receptors on osteoclasts and causes them to contract, shrinking their ability to move around and resorb bone. Less bone breakdown means less calcium flooding into your blood. This effect is fast but temporary, diminishing after about one to two days.
In the kidneys, calcitonin reduces how much calcium gets reabsorbed from urine back into the blood. It also decreases phosphate reabsorption and promotes water loss (a mild diuretic effect). The net result is that more calcium leaves the body through urine instead of being recycled.
Calcitonin vs. Parathyroid Hormone
Your body maintains blood calcium within a narrow range, and it uses two opposing hormones to do it. Parathyroid hormone (PTH) is the dominant player. When calcium drops too low, PTH kicks in to pull calcium from bones, increase calcium absorption from food, and tell your kidneys to hold onto more calcium. Calcitonin does the opposite on every front: it protects bone from being broken down, tells the kidneys to let calcium go, and brings blood levels down.
In practice, PTH tends to have a larger influence on day-to-day calcium regulation in adults. Calcitonin’s role is more pronounced during situations where calcium spikes quickly, such as after a calcium-rich meal or during rapid bone turnover in childhood and pregnancy. People who have had their thyroid removed (and therefore produce no calcitonin) generally maintain normal calcium levels through PTH and other mechanisms, which tells us calcitonin is helpful but not the sole regulator.
Calcitonin as a Medical Treatment
Because calcitonin can slow bone loss and lower blood calcium, a synthetic version derived from salmon has been used as a medication. Salmon calcitonin is roughly 50 times more potent than the human form, which makes it more practical for treatment.
For dangerously high blood calcium (hypercalcemia), calcitonin is one of the fastest-acting options available. It begins working within about two hours of the first dose, with peak effects at 24 to 48 hours and a duration of action lasting four to seven days in most cases. This speed makes it useful in hospital settings while slower, longer-lasting treatments take effect.
Calcitonin was previously available as a daily nasal spray for postmenopausal osteoporosis, delivering 200 international units per spray to help increase spinal bone mineral density. However, this use has been significantly scaled back. In 2012, the European Medicines Agency reviewed all available clinical trial data and found that long-term use of calcitonin was associated with a 0.7% to 2.4% higher risk of cancer compared to placebo, with higher rates seen in patients using the nasal spray. The agency concluded that for osteoporosis, the limited fracture-prevention benefits did not justify that risk, and recommended withdrawing the nasal spray formulation. Calcitonin remains available for short-term use in other situations, like acute hypercalcemia and Paget’s disease of bone, where the benefits are clearer and the treatment duration is shorter.
Calcitonin as a Diagnostic Marker
Because calcitonin is made by the C cells of the thyroid, it doubles as a valuable blood test for a specific type of thyroid cancer. Medullary thyroid carcinoma (MTC) originates in those same C cells, and tumors often produce excess calcitonin. Measuring calcitonin levels in the blood can help detect MTC early, especially in people who have thyroid nodules.
At a threshold of 10 pg/mL, a baseline calcitonin blood test catches essentially 100% of medullary thyroid cancers, with a specificity around 97%. Levels above 100 pg/mL are a strong signal that further investigation or surgery is warranted. For people already diagnosed with MTC, calcitonin levels are tracked over time to monitor whether the cancer is growing, shrinking, or returning after treatment.
What Abnormal Calcitonin Levels Mean
Elevated calcitonin in a blood test most commonly raises concern about medullary thyroid carcinoma, but it can also increase in other situations: kidney failure (since the kidneys help clear calcitonin from the blood), certain other cancers that produce calcitonin ectopically, and conditions that cause chronic inflammation or elevated calcium.
Low calcitonin levels, on the other hand, rarely cause noticeable problems. Even after a total thyroidectomy, when calcitonin production drops to near zero, most people maintain healthy calcium balance through other hormonal pathways. This is one reason calcitonin is sometimes described as a “fine-tuning” hormone rather than a critical one in adult life.