Calcitonin is a hormone that lowers calcium levels in your blood. It does this primarily by slowing the breakdown of bone, which is the body’s largest calcium reservoir. Produced by specialized cells in the thyroid gland, calcitonin acts as a counterbalance to parathyroid hormone (PTH), which raises blood calcium. Together, these two hormones keep calcium within a tight, functional range.
Where Calcitonin Comes From
Your thyroid gland contains two types of hormone-producing cells. The follicular cells make the thyroid hormones T3 and T4, which regulate metabolism. Scattered between them are parafollicular cells, commonly called C-cells because their primary job is calcitonin production. When calcium in your blood rises above its normal range, C-cells detect the change and release calcitonin into the bloodstream. Other signals can also trigger release (the digestive hormone gastrin is one), but elevated blood calcium is the main stimulus.
How It Protects Your Bones
The most important target of calcitonin is a type of cell called an osteoclast. Osteoclasts are responsible for breaking down bone tissue, a normal process that releases stored calcium and phosphate into the blood. Calcitonin binds to receptors on the surface of osteoclasts and essentially tells them to stop working. Once calcitonin attaches to these receptors, it triggers changes inside the cell that halt its movement and alter its structure, making the osteoclast unable to continue dissolving bone.
This effect is direct and fast. Calcitonin also causes the osteoclast to reduce the number of calcitonin receptors on its own surface over time, which is part of why the hormone’s bone-protective effect can diminish with prolonged exposure. Still, in the short term, calcitonin is one of the body’s most immediate tools for pulling the brakes on bone loss.
Effects on the Kidneys and Gut
Beyond bone, calcitonin influences how your kidneys handle minerals. It increases urinary excretion of calcium, sodium, and phosphate, meaning your kidneys flush more of these substances out rather than recycling them back into the blood. Specific calcitonin receptors have been identified along the kidney’s filtration system, though researchers still debate how significant this kidney effect is for day-to-day calcium regulation compared to its effect on bone.
Calcitonin also appears to play a role in calcium absorption from food. Animal studies have shown that calcitonin can boost levels of the active form of vitamin D, which in turn increases how much calcium the intestines absorb. In one study, calcitonin raised blood calcium by roughly 50% in animals eating a normal diet, an effect that largely disappeared when calcium was removed from the diet, pointing to intestinal absorption as the mechanism. This may seem contradictory for a hormone known to lower blood calcium, but it reflects the body’s layered approach to mineral balance: calcitonin can simultaneously slow bone breakdown and help the gut absorb dietary calcium to replenish stores through a safer route.
Calcitonin vs. Parathyroid Hormone
Calcitonin and parathyroid hormone (PTH) form an opposing pair. When blood calcium drops too low, the parathyroid glands release PTH, which activates osteoclasts to break down bone and signals the kidneys to hold onto calcium rather than excrete it. When blood calcium climbs too high, the thyroid’s C-cells release calcitonin, which does the opposite: it shuts down osteoclasts and tells the kidneys to let calcium go. Vitamin D and blood phosphate levels also feed into this system, but the PTH-calcitonin axis is the core seesaw.
One notable asymmetry exists. PTH is considered essential for survival. People who lose parathyroid function need lifelong treatment. Calcitonin, by contrast, appears to be less critical in adults. Patients who have had their thyroid completely removed (and therefore produce no calcitonin) generally maintain normal calcium levels, suggesting that other regulatory mechanisms compensate effectively. Calcitonin may matter more during periods of high calcium demand, such as pregnancy, growth, or after a calcium-rich meal, than it does at baseline.
Normal Blood Levels
In healthy adults, calcitonin circulates at very low concentrations, generally below 10 pg/mL. Women typically have lower levels than men, with a reference cutoff around 3.6 pg/mL regardless of smoking status. For men, reference values range from about 5.7 pg/mL in nonsmokers up to 7.9 pg/mL in current smokers. Smoking is one of the few lifestyle factors known to raise baseline calcitonin.
Calcitonin as a Diagnostic Marker
Because calcitonin is produced almost exclusively by thyroid C-cells, an abnormally high level can signal that something is wrong with those cells. Medullary thyroid carcinoma (MTC), a cancer that arises from C-cells, causes calcitonin levels to climb far above normal. Basal calcitonin concentrations above 60 to 100 pg/mL are highly suggestive of MTC. Levels between the normal cutoff and 60 pg/mL fall into a gray zone where either early-stage cancer or a benign overgrowth of C-cells could be the cause. For this reason, calcitonin blood tests are sometimes used to screen thyroid nodules or monitor patients with a family history of MTC.
Therapeutic Uses
A synthetic version of calcitonin, derived from salmon (which produces a more potent form than the human version), has been used as a medication for over 30 years. It is prescribed primarily for two conditions. In postmenopausal osteoporosis, salmon calcitonin helps reduce the risk of spinal fractures by slowing the osteoclast-driven bone loss that accelerates after menopause. It is typically delivered as a nasal spray. In Paget’s disease, a condition where bone remodeling becomes chaotic and excessive, calcitonin helps calm the overactive osteoclasts driving the disorder.
Salmon calcitonin has also shown potential for protecting joint cartilage in osteoarthritis, though this use is less established. An oral formulation has been developed to make the treatment more convenient and expand its applications. Calcitonin-based therapies are generally considered second-line options today, as newer osteoporosis drugs have taken a more prominent role, but they remain useful for specific patients.