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The Biology of FHH1: A Higher Internal Setting

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Familial Hypocalciuric Hypercalcemia Type 1 (FHH1) is a benign genetic condition caused by a mutation in the CASR gene. This makes the body's calcium sensors less sensitive, acting like a faulty thermostat that creates a naturally higher, but stable, set point for blood calcium.

Key Takeaways

  • • FHH1 is caused by a mutation in the CASR gene that makes the body's calcium sensors less sensitive.
  • • The condition acts like a faulty thermostat, establishing a higher natural set point for your blood calcium levels.
  • • Because the kidneys also have dull calcium sensors, they reabsorb calcium into the blood, leading to very low calcium levels in the urine.
  • • Unlike other causes of high calcium, FHH1 is present from birth and is generally considered a benign condition that does not cause symptoms.

Understanding Familial Hypocalciuric Hypercalcemia Type 1 (FHH1) starts with understanding how your body “senses” the nutrients in your blood. For most people, the body has a very strict range for calcium, but in FHH1, your body’s internal settings are simply adjusted to a different level [1].

The Body’s Calcium Thermostat

In a typical home, a thermostat monitors the air temperature. When it gets too cold, the heat turns on; when it’s warm enough, the heat shuts off. Your body has a similar “calcium thermostat” called the Calcium-Sensing Receptor (CaSR), a protein that acts as a sensor [2].

These receptors are primarily located in two places:

  1. The Parathyroid Glands: These glands act like the “furnace.” They release Parathyroid Hormone (PTH), which raises blood calcium levels [3][4].
  2. The Kidneys: These act like the “exhaust vent.” They decide how much calcium to release into your urine and how much to keep in your blood [5][4].

What is a Heterozygous Loss-of-Function Mutation?

The instructions for building these receptors are found in the CASR gene. We all inherit two copies of this gene—one from each parent.

In FHH1, there is a heterozygous loss-of-function mutation [1][6].

  • Heterozygous means that only one of your two CASR genes has the mutation; the other copy is perfectly normal [7][8].
  • Loss-of-function means the receptor built from that gene doesn’t work quite as well as it should. It is “less sensitive” to the calcium floating in your blood [6][7].

A Less Sensitive Sensor

Because your calcium sensors are less sensitive, they don’t “see” the calcium in your blood very well. It’s like having a thermostat that thinks 70∘F70^\circ\text{F} is actually 60∘F60^\circ\text{F}—it will keep the heat running until the room reaches 80∘F80^\circ\text{F} [1][6].

This leads to two hallmark features of FHH1:

  1. Hypercalcemia (High Blood Calcium): Your parathyroid glands don’t realize there is already enough calcium in your blood. They keep releasing PTH (or stay “inappropriately normal”) until the calcium reaches a higher level that the “dull” sensor can finally detect [2][9].
  2. Hypocalciuria (Low Urine Calcium): Your kidneys also have these dull sensors. Because they don’t realize the blood calcium is high, they continue to reabsorb calcium back into the blood instead of dumping it into the urine [10][6]. This is why your urine calcium levels are usually very low compared to people with other types of high calcium [6][11].

The Three Types of FHH

While FHH1 (caused by the CASR gene) is the most common form, researchers have identified two other types that involve different parts of the same sensing system [12]:

Type Gene Involved Role in the “Thermostat”
FHH1 CASR The actual sensor that detects calcium [2].
FHH2 GNA11 The “wiring” that sends the signal from the sensor to the cell [13][14].
FHH3 AP2S1 The machinery that moves the sensors in and out of the cell wall [13][15].

FHH1 is typically the most “benign” (harmless) version, whereas FHH3 can sometimes lead to slightly higher calcium levels and may occasionally be associated with learning or behavioral challenges [16][12]. Knowing you have the CASR (FHH1) version is often a source of relief, as it confirms your body is simply working with a higher, but stable, internal setting [1][17].

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Frequently Asked Questions

What causes Familial Hypocalciuric Hypercalcemia Type 1 (FHH1)?
FHH1 is caused by a heterozygous loss-of-function mutation in the CASR gene. This means you inherited one normal copy and one altered copy of the gene, which makes the calcium sensors in your body less sensitive to the calcium in your blood.
Why is my blood calcium high if I have FHH1?
In FHH1, your calcium sensors act like a thermostat that thinks your blood calcium is lower than it actually is. Your parathyroid glands keep releasing hormone until your calcium reaches a higher level that your less sensitive sensors can finally detect.
Why is low urine calcium a sign of FHH1?
Your kidneys use the same calcium sensors as your parathyroid glands. Because these sensors are dull in FHH1, your kidneys do not realize your blood calcium is high and continue to reabsorb calcium back into your blood instead of filtering it into your urine.
Will I pass FHH1 on to my children?
Because FHH1 is a heterozygous mutation, meaning it affects only one copy of the gene, there is a 50 percent chance of passing the altered CASR gene with the higher calcium setting on to each of your children.
Is FHH1 dangerous to my health?
FHH1 is generally considered a benign or harmless condition. Knowing you have the CASR gene mutation confirms your body is simply working with a higher, stable internal setting, and most people never experience symptoms or health problems from it.

Questions for Your Doctor

  • • Does my specific genetic mutation mean my calcium 'set point' is mildly or significantly higher than average?
  • • Is my Parathyroid Hormone (PTH) level 'inappropriately normal' for the amount of calcium in my blood?
  • • Can you explain my Calcium-to-Creatinine Clearance Ratio (CCCR) in the context of my kidneys holding onto calcium?
  • • Since this is a heterozygous mutation, what is the 50% chance that I could pass this 'high thermostat' setting on to my children?

Questions for You

  • • How long has my calcium been high? (FHH1 is present from birth, while other conditions often develop later in life).
  • • Do I have any biological relatives who were diagnosed with high calcium but never had symptoms or health problems because of it?
  • • Before I knew about my blood test results, did I feel like my body was functioning normally?

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This page explains the biology and genetics of FHH1 for educational purposes only. Always consult your endocrinologist or genetic counselor to discuss your specific lab results, genetic tests, and diagnosis.

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