Medical Genetics · Familial Hypomagnesemia with Secondary Hypocalcemia

The Biology of HSH: The Magnesium and Calcium Connection

At a Glance

HSH is an inherited TRPM6-related disorder that prevents the gut from absorbing enough magnesium. Severe magnesium deficiency disrupts the parathyroid glands and causes low calcium, which often improves when magnesium is medically replaced.

Understanding the biology of Familial hypomagnesemia with secondary hypocalcemia (HSH) helps explain why your child’s symptoms were so severe and why the diagnosis was likely difficult to reach. At its core, HSH is a “transport” problem—the body has plenty of magnesium in the diet, but it lacks the necessary machinery to properly move it from the gut into the bloodstream ^[1]^[2].

The TRPM6 Channel

The primary cause of HSH is a mutation in the TRPM6 gene ^[3]. In a healthy body, this gene provides the instructions for building a specialized channel in the lining of the small intestine and the colon, as well as the kidneys ^[4].

When your child consumes magnesium through milk or food, these TRPM6 channels actively absorb the mineral into the blood. In HSH, this active intestinal absorption is severely impaired ^[5]. Because the body cannot actively absorb enough magnesium, blood levels of the mineral drop to dangerously low levels, often called profound hypomagnesemia ^[6].

The Cascade: Why Calcium Also Drops

One of the most confusing parts of HSH is that it doesn’t just affect magnesium; it also causes very low calcium levels (hypocalcemia) ^[1]. This happens because magnesium is essential for the function of the parathyroid glands, which are responsible for maintaining calcium balance ^[3].

When magnesium is severely low, two things go wrong with the parathyroid system:

Hormone Suppression: The parathyroid glands stop releasing adequate parathyroid hormone (PTH), the chemical signal that tells the body to increase calcium ^[7].
Organ Resistance: Even if some PTH is released, the rest of the body (like the bones and kidneys) becomes resistant to its signals because magnesium is required for those organs to respond ^[7]^[8].

This double failure is why HSH can look like a primary calcium disorder when it is actually a magnesium problem at its root. Once magnesium levels are medically corrected, the parathyroid system typically recovers, and calcium levels usually normalize ^[9]^[10].

Common Misdiagnoses

Because HSH is ultra-rare and its symptoms are complex, it is frequently mistaken for other conditions. Clinicians interpret magnesium, calcium, PTH, phosphate, vitamin D, kidney findings, treatment history, and genetics together:

Condition	Why it’s confused with HSH	Key Difference
Primary Epilepsy	Both involve frequent, severe seizures in infants ^[11].	While magnesium replacement treats the underlying electrolyte trigger, a seizure emergency still requires standard seizure first aid and clinician-directed antiseizure treatment. Do not delay emergency services waiting for oral magnesium to work ^[12].
Primary Hypoparathyroidism	Both show low calcium and low/normal PTH levels ^[13].	In primary hypoparathyroidism, magnesium is often normal. In HSH, magnesium is extremely low ^[1].
Vitamin D Deficiency	Both can cause low calcium and “soft” bones (rickets) ^[1].	Vitamin D levels in children with HSH are usually normal or high, which helps support the distinction ^[1].

Inheriting HSH

HSH is an autosomal recessive condition ^[5]. This means that for a child to have the disorder, they must inherit one mutated copy of the TRPM6 gene from each parent ^[3].

Parents of a child with HSH are typically “carriers”—they have one working gene and one mutated gene. Carriers usually have perfectly normal magnesium levels and no symptoms because their one working gene is enough to handle the body’s needs ^[14]^[2]. When two carriers have a child, there is a 25% (1 in 4) chance with each pregnancy that the child will inherit both mutated genes and have HSH ^[5]. Knowing this allows families to work with genetic counselors to plan for the health of future children, who can be screened and treated ^[13].

Common questions in this guide

What causes familial hypomagnesemia with secondary hypocalcemia?

HSH is caused by disease-causing changes in both copies of the TRPM6 gene. These changes severely reduce active magnesium absorption in the intestine, so blood magnesium becomes very low and calcium falls secondarily.

Why does low magnesium make calcium levels drop?

Severe magnesium deficiency can stop the parathyroid glands from releasing enough parathyroid hormone and can make the body less responsive to the hormone. After magnesium is corrected, parathyroid function usually recovers and calcium levels often return toward normal.

How is HSH distinguished from epilepsy or other causes of low calcium?

Clinicians compare magnesium, calcium, parathyroid hormone, phosphate, vitamin D, kidney findings, treatment history, and genetic testing. Extremely low magnesium supports HSH over primary hypoparathyroidism, while vitamin D levels are usually normal or high; seizures still require standard emergency care and clinician-directed treatment.

How is HSH inherited, and what is the chance it will affect another child?

HSH is autosomal recessive, meaning an affected child inherits one altered TRPM6 copy from each parent. If both parents are carriers, each pregnancy has a 25% chance of HSH, a 50% chance of being a carrier, and a 25% chance of inheriting neither altered copy.

What does a fractional excretion of magnesium test tell us in HSH?

The fractional excretion of magnesium test estimates how much magnesium the kidneys are losing compared with how much they filter. It helps clinicians determine whether the kidneys are conserving magnesium and should be interpreted alongside blood levels, urine results, and genetic findings.

Can genetic testing miss a TRPM6 mutation?

The ability of testing to detect TRPM6 changes depends on which methods are included. Ask the genetics team whether the evaluation covered uncommon splice-site changes and larger deletions, as well as standard sequence changes.

Get notified when new evidence is published on Primary hypomagnesemia with secondary hypocalcemia.

We monitor PubMed for new peer-reviewed studies on this topic and email a short summary when something meaningful changes.

References (14)

1
Clinical Spectrum of Primary Hypomagnesemia with Secondary Hypocalcemia due to TRPM6 Mutation.
Aftab S, Anjum MN, Aslam N, et al.
Hormone research in paediatrics 2026; (99(7)):1045-1050 doi:10.1159/000547303.
PMID: 40853861
2
Intestinal hypomagnesemia in an Iranian patient with a novel TRPM6 variant: a case report and review of the literature.
Kamali F, Jamee M, Sayer JA, et al.
CEN case reports 2023; (12(4)):413-418 doi:10.1007/s13730-023-00785-8.
PMID: 36967423
3
Case Report: A Novel Non-Canonical Splice Site Variant (c.1638+7T>C) in TRPM6 Cause Primary Homagnesemia With Secondary Hocalcemia.
Song J, Lei J, Zhang J, et al.
Frontiers in pediatrics 2022; (10()):834241 doi:10.3389/fped.2022.834241.
PMID: 35692977
4
Magnesium Homeostasis: Lessons from Human Genetics.
Morrison AR
Clinical journal of the American Society of Nephrology : CJASN 2023; (18(7)):969-978 doi:10.2215/CJN.0000000000000103.
PMID: 36723340
5
Case Report: Novel TRPM6 Mutations Cause Hereditary Hypomagnesemia With Secondary Hypocalcemia in a Chinese Family and a Literature Review.
Han Y, Zhao Y, Wang H, Huo L
Frontiers in pediatrics 2022; (10()):912524 doi:10.3389/fped.2022.912524.
PMID: 35903165
6
Three cases of hereditary hypomagnesemia with secondary hypocalcemia caused by novel TRPM6 gene mutations and a literature review.
Yang M, Zheng Z, Shan X, Luo Y
Journal of pediatric endocrinology & metabolism : JPEM 2026; (39(9)):917-925 doi:10.1515/jpem-2026-0185.
PMID: 42324811
7
Delayed recognition of hypomagnesemia causing refractory Hypocalcemia after Reoperative neck surgery in a resource-limited setting. a case report.
Flores CS, Vásquez SM, Pineda A, et al.
Oxford medical case reports 2026; (2026(7)):omag132 doi:10.1093/omcr/omag132.
PMID: 42516777
8
A Case of Renal Tubular Acidosis with Sjogren's Syndrome Showing Paradoxical Block of PTH Due to Severe Hypomagnesemia.
Kumar BV, Sivalingam M, Kumaran GS, Balakrishnan B
Indian journal of clinical biochemistry : IJCB 2017; (32(4)):496-499 doi:10.1007/s12291-016-0629-y.
PMID: 29062185
9
Renal magnesium wasting and functional hypoparathyroidism: a diagnostic and therapeutic challenge in hypocalcemia.
Schlegel A
Laboratory medicine 2026; (57(4)) doi:10.1093/labmed/lmag035.
PMID: 42290042
10
Suppression of Parathyroid Hormone in a Patient with Severe Magnesium Depletion.
Mutnuri S, Fernandez I, Kochar T
Case reports in nephrology 2016; (2016()):2608538 doi:10.1155/2016/2608538.
PMID: 27190662
11
Treatment Difficulties in Hypomagnesemia Secondary to the Transient Receptor Potential Melastatin 6 Gene: A Case Report with Novel Mutation
Yücel H, Genç Sel Ç, Kasapkara ÇS, et al.
Journal of clinical research in pediatric endocrinology 2021; (13(1)):114-118 doi:10.4274/jcrpe.galenos.2020.2020.0004.
PMID: 32302086
12
Hereditary hypomagnesemia with secondary hypocalcemia caused by a novel mutation in TRPM6 gene.
Dokurel Çetin İ, Betül Gerik-Çelebi H, Demiral M, Çetin O
Journal of pediatric endocrinology & metabolism : JPEM 2024; (37(2)):184-188 doi:10.1515/jpem-2023-0378.
PMID: 38084506
13
Hypomagnesemia and functional hypoparathyroidism due to novel mutations in the Mg-channel TRPM6.
Astor MC, Løvås K, Wolff AS, et al.
Endocrine connections 2015; (4(4)):215-22 doi:10.1530/EC-15-0066.
PMID: 26273099
14
Familial hypomagnesemia with secondary hypocalcemia: a novel TRPM6 mutation associated with treatment-resistant migraine in a North African family.
Paola PM, Gianni C, Emanuele M, et al.
Neurological sciences : official journal of the Italian Neurological Society and of the Italian Society of Clinical Neurophysiology 2026; (47(10)).
PMID: 42830330

This page explains the biology, diagnosis, and inheritance of HSH for educational purposes only and does not constitute medical advice. Your child’s clinician and genetic counselor should interpret laboratory or genetic results and recommend care.