Calcitonin
Calcitonin (also known as thyrocalcitonin) is a 32-amino acid linear polypeptide hormone that is produced
in humans primarily by the parafollicular cells (also known as C-cells) of the thyroid gland, and in many other animals in the ultimopharyngeal body. It acts to reduce blood calcium (Ca2+), opposing the effects of parathyroid hormone (PTH).
Calcitonin has been found in fish, reptiles, birds, and mammals. Its importance in humans has not been as well established as its importance in other animals, as its function is usually not significant in the regulation of normal calcium homeostasis. It belongs to the calcitonin-like protein family.

# Biosynthesis and regulation
Calcitonin is formed by the proteolytic cleavage of a larger prepropeptide, which is the product of the CALC1 gene (CALCA). It is functionally an antagonist with PTH and Vitamin D3.The CALC1 gene belongs to a superfamily of related protein hormone precursors including islet amyloid precursor protein, calcitonin gene-related peptide, and the precursor of adrenomedullin.
Secretion of calcitonin is stimulated by:
- an increase in serum 
- gastrin and pentagastrin.

# Function
The hormone participates in calcium (Ca2+) and phosphorus metabolism. In many ways, calcitonin counteracts parathyroid hormone (PTH) and vitamin D
More specifically, calcitonin lowers blood Ca2+ levels in two ways:
- Major effect: Inhibits osteoclast activity in bones
- Minor effect: Inhibits renal tubular cell reabsorption of Ca2+ and phosphate, allowing them to be excreted in the urine
High concentrations of calcitonin may be able to increase urinary excretion of calcium and phosphate via the renal tubules. leading to marked hypocalcemia. However, this is a minor effect with no physiological significance in humans. It is also a short-lived effect because the kidneys become resistant to calcitonin, as demonstrated by the kidney's unaffected excretion of calcium in patients with thyroid tumors that secrete excessive calcitonin.
In its skeleton-preserving actions, calcitonin protects against calcium loss from skeleton during periods of calcium mobilization, such as pregnancy and, especially, lactation. The protective mechanisms include the direct inhibition of bone resorption and the indirect effect through the inhibition of the release of prolactin from the pituitary gland. The reason provided is that prolactin induces the release of PTH related peptide which enhances bone resorption, but is still under investigation.,
Other effects are in preventing postprandial hypercalcemia resulting from absorption of Ca2+. Also, calcitonin inhibits food intake in rats and monkeys, and may have CNS action involving the regulation of feeding and appetite.
Calcitonin lowers blood calcium and phosphorus mainly through its inhibition of osteoclasts. Osteoblasts do not have calcitonin receptors and are therefore not directly affected by calcitonin levels. However, since bone resorption and bone formation are coupled processes, eventually calcitonin's inhibition of osteoclastic activity leads to increased osteoblastic activity (as an indirect effect).

# Receptor
The calcitonin receptor, localized to osteoclasts, the kidney, and regions of the brain, is a G protein-coupled receptor. It is coupled by Gs to adenylate cyclase, and thereby to the generation of cAMP in target cells. It may also affect the ovaries in women and the testes in men.

# Discovery
Calcitonin was purified in 1962 by Copp and Cheney. While it was initially considered a secretion of the parathyroid glands, it was later identified as the secretion of the C-cells of the thyroid gland.

# Medical Significance
Calcitonin assay is used in identifying patients with nodular thyroid diseases. It is helpful in making an early diagnosis of medullary carcinoma of thyroid. A malignancy of the parafollicular cells, i.e. Medullary thyroid cancer, typically produces an elevated serum calcitonin level. Prognosis of MTC depends on early detection and treatment.

# Pharmacology
Salmon calcitonin is used for the treatment of:
- Postmenopausal osteoporosis
- Hypercalcaemia
- Paget's disease
- Bone metastases
- Phantom limb pain
It has been investigated as a possible non-operative treatment for spinal stenosis.
The following information is from the UK Electronic Medicines Compendium

## General characteristics of the active substance
Salmon calcitonin is rapidly absorbed and eliminated. Peak plasma concentrations are attained within the first hour of administration.
Animal studies have shown that calcitonin is primarily metabolised via proteolysis in the kidney following parenteral administration. The metabolites lack the specific biological activity of calcitonin. Bioavailability following subcutaneous and intramuscular injection in humans is high and similar for the two routes of administration (71% and 66%, respectively).
Calcitonin has short absorption and elimination half-lives of 1015 minutes and 5080 minutes, respectively. Salmon calcitonin is primarily and almost exclusively degraded in the kidneys, forming pharmacologically inactive fragments of the molecule. Therefore, the metabolic clearance is much lower in patients with end-stage renal failure than in healthy subjects. However, the clinical relevance of this finding is not known.  Plasma protein binding is 30% to 40%.

## Characteristics in patients
There is a relationship between the subcutaneous dose of calcitonin and peak plasma concentrations. Following parenteral administration of 100 IU calcitonin, peak plasma concentration lies between about 200 and 400 pg/ml. Higher blood levels may be associated with increased incidence of nausea, vomiting, and secretory diarrhea.

## Preclinical safety data
Conventional long-term toxicity, reproduction, mutagenicity, and carcinogenicity studies have been performed in laboratory animals. Salmon calcitonin is devoid of embryotoxic, teratogenic, and mutagenic potential.
An increased incidence of pituitary adenomas has been reported in rats given synthetic salmon calcitonin for 1 year. This is considered a species-specific effect and of no clinical relevance. Salmon calcitonin does not cross the placental barrier.
In lactating animals given calcitonin, suppression of milk production has been observed. Calcitonin is secreted into the milk.

# Pharmaceutical manufacture
Calcitonin was extracted from the ultimobranchial glands (thyroid-like glands) of fish, particularly salmon.  Salmon calcitonin resembles human calcitonin, but is more active.  At present, it is produced either by recombinant DNA technology or by chemical peptide synthesis.  The pharmacological properties of the synthetic and recombinant peptides have been demonstrated to be qualitatively and quantitatively equivalent.

# Uses of calcitonin

## Treatments
Calcitonin can be used therapeutically for the treatment of hypercalcemia or osteoporosis.
In a recent clinical study, subcutaneous injections of calcitonin have reduced the incidence of fractures and reduced the decrease in bone mass in women with type 2 diabetes complicated with osteoporosis.
Subcutaneous injections of calcitonin in patients suffering from mania resulted in significant decreases in irritability, euphoria and hyperactivity and hence calcitonin holds promise for treating bipolar disorder.  However no further work on this potential application of calcitonin has been reported.

## Diagnostics
It may be used diagnostically as a tumor marker for medullary thyroid cancer, in which high calcitonin levels may be present and elevated levels after surgery may indicate recurrence. It may even be used on biopsy samples from suspicious lesions (e.g., lymph nodes that are swollen) to establish whether they are metastases of the original cancer.
Cutoffs for calcitonin to distinguish cases with medullary thyroid cancer have been suggested to be as follows, with a higher value increasing the suspicion of medullary thyroid cancer:
- females: 5 ng/L or pg/mL
- males: 12 ng/L or pg/mL
- children under 6 months of age: 40 ng/L or pg/mL
- children between 6 months and 3 years of age: 15 ng/L or pg/mL
When over 3 years of age, adult cutoffs may be used
Increased levels of calcitonin have also been reported for various other conditions. They include: C-cell hyperplasia, nonthyroidal oat cell carcinoma, nonthyroidal small cell carcinoma and other nonthyroidal malignancies, acute and chronic renal failure, hypercalcemia, hypergastrinemia and other gastrointestinal disorders, and pulmonary disease.

## Structure
Calcitonin is a polypeptide hormone of 32 amino acids, with a molecular weight of 3454.93 daltons. Its structure comprises a single alpha helix. Alternative splicing of the gene coding for calcitonin produces a distantly related peptide of 37 amino acids, called calcitonin gene-related peptide (CGRP), beta type.
The following are the amino acid sequences of salmon  and human calcitonin:
- salmon:
- human:
Compared to salmon calcitonin, human calcitonin differs at 16 residues.