Parathyroid hormone

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Parathyroid hormone


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USMLE® Step 1 questions

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High Yield Notes

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Parathyroid hormone

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USMLE® Step 1 style questions USMLE

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Parathyroid hormone is released from the parathyroid gland in response to hypocalcemia. Which of the following electrolyte changes will be seen in this patient?

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Parathyroid hormone (PTH) p. 336

bone disorders p. 473

bone formation p. 471

calcitonin and p. 342

in hyperparathyroidism p. 344

kidney effects p. 614

nephron physiology p. 609

osteomalacia/rickets p. 473

Paget disease of bone p. 473

pseudohypoparathyroidism and p. 350

signaling pathways of p. 353

thymic aplasia p. 114

vitamin D and p. 336


The body’s blood calcium level stays stable thanks to three hormones: parathyroid hormone, vitamin D, and calcitonin.

Parathyroid hormone and vitamin D help increase calcium levels, whereas calcitonin helps lower them. Let’s focus on the role of parathyroid hormone.

The majority of the extracellular calcium, the calcium in the blood and interstitium, is split almost equally into calcium that’s diffusible and calcium that’s not diffusible.

Diffusible calcium is small enough to diffuse across cell membranes and there are two subcategories.

The first is free-ionized calcium, which is involved in all sorts of cellular processes like neuronal action potentials, contraction of skeletal, smooth, and cardiac muscle, hormone secretion, and blood coagulation, all of which are tightly regulated by enzymes and hormones.

The second category is complexed calcium, which is where the positively charged calcium is ionically linked to tiny negatively charged molecules like oxalate and phosphate, which are small anions, that are found in our blood.

The complexed calcium forms a molecule that’s electrically neutral but unlike free-ionized calcium it’s not useful for cellular processes.

Finally there’s the non-diffusible calcium which is bound to large negatively charged proteins like albumin.

The resulting protein-calcium complex is too large and charged to cross membranes, so the non-diffusible calcium is also uninvolved in cellular processes.

Now blood calcium is regulated mainly by parathyroid hormone.

Parathyroid hormone, or PTH, comes from the parathyroid glands, which are 4 pea sized glands buried within the posterior part of the thyroid gland.

Inside the parathyroid glands are parathyroid or chief cells which synthesize a protein called preproPTH in their endoplasmic reticulum.

This long protein chain is 115 amino acid long and contains the parathyroid hormone segment, but also a “pre” segment and a “pro” segment.


Parathyroid hormone (PTH) is a hormone produced by the parathyroid glands and plays a crucial role in regulating the balance of calcium in the body. This hormone is released when the calcium-sensing receptor on parathyroid cells detects a decrease in extracellular calcium.

PTH increases the level of calcium in the blood, by stimulating the bones to release calcium, increasing the absorption of calcium from food in the intestines, and decreasing the excretion of calcium by the kidneys.


  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Principles of Anatomy and Physiology" Wiley (2014)
  4. "Human Anatomy & Physiology" Pearson (2018)
  5. "Calcium Signaling" Cell (2007)
  6. "Activation Mechanism for CRAC Current and Store-operated Ca2+Entry" Journal of Biological Chemistry (2006)

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