Hypophosphataemic rickets

[Phloston]

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Hypophosphataemic rickets is X-linked dominant.

Apparently rickets/osteomalacia occurs due to decreased PCT HCO3- reabsorption.

My question: how would this affect PTH and serum calcium levels?

Cheers,

 

[tzmd]

The problem here is an abnormally low vitamin D concentration in blood serum.

Ca: decreased and PTH: elevated.

 

[thinker]

It doesn't have to be x-linked dominant btw or even hereditary for that matter.

This might be beyond Step1 but I happened to see a case of Hypophosphatemic rickets just this week in the Pediatrics outpatient department. The young boy was my patient and had rickets that was refractory to treatment by oral replacement of phosphates. The doctors have been looking for a long time for a tumor but still can't find it. Due to the negative family history (so it's not X-linked dominant or autosomal dominant) they are sure that the diagnosis is Tumor-induced hypophosphatemia (or Osteomalacia).

The tumor produces FGF23 (which is involved in the autosomal dominant form) and this prevents the translocation of Na-P pump in the kidney and leads to hypophosphatemia. It also prevents Vitamin D function (not sure how)

So it's like a paraneoplastic disease. Unfortunately for this kid however they still can't find the tumor as apparently it can be very small and in unusual locations.

Just wanted to share this as it's one of the more interesting cases that we got to see.

 

[Dallas]

Hypophosphataemic rickets is X-linked dominant.

Apparently rickets/osteomalacia occurs due to decreased PCT HCO3- reabsorption.

My question: how would this affect PTH and serum calcium levels?

Cheers,

Your description of the disease sounds scarily similar to Renal Tubular Acidosis type 2.

 

[Phloston]

That is interesting. Thanks for sharing that.

 

[glykon]

The problem here is an abnormally low vitamin D concentration in blood serum.

Ca: decreased and PTH: elevated.

Yap, agree. And also the increased excretion of phosphate (mutated PHEX gene as per Wiki 😀) which impairs bone mineralization.
Phloston, where do they say it's due to decreased reabs of HCO3-?

 

[Phloston]

Yap, agree. And also the increased excretion of phosphate (mutated PHEX gene as per Wiki 😀) which impairs bone mineralization.
Phloston, where do they say it's due to decreased reabs of HCO3-?

Hypophosphataemic rickets was an incorrect answer choice for one of the questions on Kaplan QBank. In the explanation, they basically just gave that one-liner. Then my mind just went off on a tangent.

 

[glykon]

It doesn't have to be x-linked dominant btw or even hereditary for that matter.

This might be beyond Step1 but I happened to see a case of Hypophosphatemic rickets just this week in the Pediatrics outpatient department. The young boy was my patient and had rickets that was refractory to treatment by oral replacement of phosphates. The doctors have been looking for a long time for a tumor but still can't find it. Due to the negative family history (so it's not X-linked dominant or autosomal dominant) they are sure that the diagnosis is Tumor-induced hypophosphatemia (or Osteomalacia).

The tumor produces FGF23 (which is involved in the autosomal dominant form) and this prevents the translocation of Na-P pump in the kidney and leads to hypophosphatemia. It also prevents Vitamin D function (not sure how)

So it's like a paraneoplastic disease. Unfortunately for this kid however they still can't find the tumor as apparently it can be very small and in unusual locations.

Just wanted to share this as it's one of the more interesting cases that we got to see.

Uau, thanks for the input, it's very interesting and helpful.
This just reminds me, we had a 27yo M patient with Lynch sdr., no family history at all. When I asked our teacher about the case, he said that yes, family history is very important for these genetic ds, but let's not forget some mutations simply arise de novo...Hope that's the case (if possible) for your patient as well 🙂

 

[glykon]

Hypophosphataemic rickets was an incorrect answer choice for one of the questions on Kaplan QBank. In the explanation, they basically just gave that one-liner. Then my mind just went off on a tangent.

Is that in the Genetics section of Kaplan QBank? I'll do those qs tomorrow, so I'm curios😀

 

[ijn]

The problem here is an abnormally low vitamin D concentration in blood serum.

Ca: decreased and PTH: elevated.

With X-linked hypophosphatemic rickets you have normal levels of calcium and vitamin D, but low phosphate levels.

The problem is your vitamin D levels shouldn't just be "normal" with low phosphate levels. They should be high. The lack of the PHEX gene results in increased FGF23 levels. This factor suppress 1-alpha hydroxylase activity and thus prevents an appropriate elevation of 1,25-Vit D. Elevated FGF23 levels also result in internalization of sodium-phosphate IIa and IIc cotransporters from the renal proximal tubule.

You're far more likely to see X-linked hypophosphatemic rickets on a pediatrics licensing exam, STEP 2, or pediatrics clerkship NBME than on STEP 1.