There is a balance in the body between phosphorus and calcium levels.
In CKD cats,
imbalances are very common.
In the worst case, problems
then also arise with a hormone called parathyroid hormone (PTH).
These imbalances can eventually
lead to a condition called secondary hyperparathyroidism, which can have
serious effects on the cat.
It is therefore important to
monitor phosphorus, calcium and PTH levels in all CKD cats.
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Phosphorus (P or Pi)
a mineral which is essential for bodily function. Most of the body's
phosphorus is contained in bone.
Phosphorus is important for energy production, acid base balance
(imbalances in this area can lead to a condition known as
acidosis) and for delivering oxygen to the body's cells.
are responsible for removing excess phosphorus from the body, but
damaged CKD kidneys find this more difficult, so phosphorus levels
within the body begin to rise. This activates a
phosphatonin called fibroblast growth factor 23 (FGF-23), which is
important for the regulation of phosphorus levels in the blood.
FGF-23 may play a role in the early detection of CKD, as discussed in
If you are
looking at your cat's blood test results, phosphorus may be shown as
phos, P or Pi.
Virtually every CKD cat will eventually develop high phosphorus
levels (hyperphosphataemia). This happens
because the damaged kidneys can no
longer excrete excess phosphorus properly.
There is a balance in the body between phosphorus and calcium. Thus
elevated phosphorus levels can adversely affect calcium levels, with
potentially serious consequences, and can generally make the
CKD progress faster. They also make the cat feel very unwell.
Symptoms include nausea, weakness and loss of appetite.
these risks, having a phosphorus level within the normal range is not
sufficient for a CKD cat.
If your cat's phosphorus level is over 6
mg/dl (USA) or 1.9 mmol/L (international), you need to take
steps to control it.
Your aim is to reduce your cat's phosphorus to a level of
4.6 mg/dl (1.5 mmol/L
You may not
be able to get it this low if your cat is in
Stage 3 or 4, but aim never to let it go above 6 mg/dl (1.9 mmol/L international).
Low phosphorus levels are extremely rare in CKD cats, but may occasionally
arise. In most cases they tend to happen if you are over zealous with
used to control high phosphorus levels, but may also be seen in cats with
diabetes controlled by insulin.
Certain types of cancer may cause low phosphorus levels, as may feeding a cat
following a period of prolonged starvation.
Low phosphorus levels are usually not a problem, although they may sometimes
cause lethargy or anaemia, and if the level falls really low, seizures might occur.
Calcium exists in three different forms in the blood:
bound, which is bound to proteins
(around 80% to albumin and 20% to globulins)
complexed, with certain anions such
ionised, or free
The total calcium figure on a blood
chemistry panel is the total of all of these, with ionised calcium
comprising about 50% of the total. Ionised calcium is the value of main
concern for most CKD cats.
CKD cats may have elevated calcium levels (hypercalcaemia) or
reduced calcium levels (hypocalcaemia).
calcium levels are relatively common in CKD cats. In
Hypercalcemia in cats (2001)
Presentation to the World Small Animal Veterinary Association World
Congress, Dr D Chew states that 38% of hypercalcaemic cats in one
study had CKD.
The range for
calcium varies from laboratory to laboratory, but generally speaking:
a level over
11 mg/dl US (2.75 mmol/L international) is considered to be
hypercalcaemia, though you may not see any symptoms until it is higher
goes above 15 mg/dl US (3.75 mmol/Linternational), a cat may
have constipation, little appetite and sleep a lot.
approach 20 mg/dl US (5.0 mmol/L international), the cat may go into a
coma and die.
Hypercalcemia in dogs and cats (2016)
Peterson ME Merck Veterinary Manual says "serum
concentrations of >18 mg/dL are often associated with severe,
life-threatening signs." Fortunately, such extremely high levels
are very rare, so in practice you are unlikely to see them.
level of calcium in the blood is in ratio to the phosphorus level -
calcium levels are usually at an approximate level of 100 - 200% of
phosphorus levels. Since phosphorus levels are often high in CKD cats, calcium levels may
also rise in an attempt to maintain the ratio. Conversely, if elevated
phosphorus levels are reduced, often the calcium levels will also reduce to a
Other causes of
hypercalcaemia include metabolic acidosis
(since acid in the blood leads to
the release of calcium from proteins or from bone) or, rarely,
A type of
called fructooligosaccharides (FOS), often contained in
cause elevated calcium levels.
In rare cases,
excess Vitamin D in the diet causes hypercalcaemia. In 2006, Royal Canin
had to recall some foods to which too much Vitamin D had been added by
mistake, causing hypercalcaemia in four cats.
Some people have
found that using
lactulose to control constipation has led to hypercalcaemia in their cats. This might just be coincidence, but you may wish to avoid
lactulose if your cat already has hypercalcaemia, and consider alternative
constipation treatments if your cat develops hypercalcaemia while using lactulose.
Slippery elm bark also contains calcium, so it is probably safer not to use it
if your cat has hypercalcaemia.
The first thing
to do is to run the calcium test again.
Hypercalcemia in dogs and cats: etiology and diagnostic approach
(2002) Nelson R Presentation to the27th World Small Animal Veterinary
Association Congress 2002 states "Hypercalcemia
should always be reconfirmed, preferably from a nonlipemic blood sample
obtained from the dog or cat following a 12 hour fast, before embarking on
an extensive diagnostic evaluation."
If the second
test also indicates hypercalcaemia for which no obvious cause can be
found, the next step is to run an
ionised calcium test. If this is normal, there is no need to worry about the
hypercalcaemia per se. However, you do need to consider the product of
total calcium multiplied by phosphorus (see
Low Calcium Levels (Hypocalcaemia)
Calcitriol (1,25 dihydroxycholecalciferol) is the active form of Vitamin
D3, which, despite its name, is actually a hormone. It facilitates the
absorption of calcium from the gastointestinal tract and the release of
calcium from the bones where it is stored. Cats have
to make calcitriol from Vitamin D before they are able to use it, and the
final step of this process is performed by the kidneys.
Because of their failing kidneys, CKD cats may no longer be able to
convert Vitamin D into calcitriol, so they absorb less calcium from the
gastrointestinal tract and release less from the bones, and thus levels
of calcium in the body may fall.
Excess phosphorus levels may also reduce
calcium levels by suppressing the production of calcitriol.
stated above, ionised calcium is what matters for CKD cats, you do have to
take total calcium levels into account in one main regard: you have
to consider the level of
phosphorus multiplied by total calcium. This is
because cats with a high number are at risk of a problem called
Hypercalcemia in cats (2001) Chew D
Presentation to the World Small Animal Veterinary Association World
says "The degree of interaction with serum phosphorus is
important, as those with a total serum calcium times phosphorus
concentration product greater than 70 are most likely to have severe
tissue changes associated with mineralization."
As a rough
guide, if phosphorus multiplied by total calcium is higher than around 60
in US values or 5 in international values, your cat is at risk.
you can multiply phosphorus by ionised calcium, and if the level is higher
than 8.75 in US values or 35 in international values, there is a risk of
It is less
likely to be a problem if the product is above these levels because of a
higher calcium level rather than a higher phosphorus level; but I would
still recommend trying to reduce these levels if at all possible.
Phosphorus and calcium levels in the body are controlled by glands called the parathyroid glands,
which adjust phosphorus and/or calcium levels as appropriate via two
hormones called parathyroid hormone (also known as PTH) and calcitriol.
In healthy cats, if phosphorus
levels are too high, or if calcium levels are too low, the levels of
ionised calcium in the body fall. The parathyroid glands are then
stimulated to produce more parathyroid hormone,
which tries to adjust the levels of calcium and phosphorus to their
correct levels, partly by taking calcium from the bone and partly by increasing
the excretion of phosphorus in the urine. Parathyroid hormone also stimulates the
kidneys to produce calcitriol, which helps to increase the levels of ionised
calcium in the blood.
Once ionised calcium
has been restored to the correct level, this process should cease because
normally calcitriol can stop the secretion of PTH. Unfortunately, as kidney
function reduces and calcitriol is not produced in adequate amounts,
this mechanism may no longer work properly. Therefore, phosphorus levels may continue
to rise and may also block calcitriol synthesis. A vicious circle then
results, and eventually a condition called
hyperparathyroidism may develop.
speaking, problems tend to arise when PTH is around three times the level
of the top of the normal range.
Michigan State University Diagnostic Center for
Population & Animal Health
states that the
reference range for PTH is 0.4 - 2.5 pmol/L. Therefore toxicity can be expected
when PTH is 7.5 pmol/L or over. However, if it is at all elevated, it is best to
take action to reduce it as soon as possible, so as to avoid secondary
hyperparathyroidism is not the same as
caused by part of the body's mechanism for controlling phosphorus and
calcium levels effectively going into overdrive, as explained
As the problems
with the regulation of calcitriol and parathyroid stimulating hormone
(PTH) progress, levels of PTH will continue to rise and eventually
secondary hyperparathyroidism results. Blood calcium levels may appear to
be normal or low but over time various symptoms may appear, including lack
of appetite, anaemia, reduced immunity to infection and
muscle weakness. Other severe consequences may also result, see
Chronic kidney disease (CKD) in dogs and cats -
staging and management strategies (2015) A Presentation to the
Virginia Veterinary Medical Association 2015 Virginia Veterinary
Conference, Dr D Chew states that it is possible for a CKD
cat to develop secondary hyperparathyroidism even if phosphorus levels and
ionised calcium levels are normal. He explains "In the early stages of
chronic kidney disease increased levels of parathyroid hormone (PTH) keep
serum phosphorous within the normal range by increasing phosphate
excretion into urine. This allows for normalization of serum phosphorous
at the expense of hyperparathyroidism."
Veterinary Partner explains more about
secondary hyperparathyroidism in easy to understand language.
(2004) Hörl WH
Nephrology Dialysis Transplantation 19 (suppl 5)
v2-v8 discusses the problems that may arise with secondary
hyperparathyroidism in humans.
hyperparathyroidism progresses, the cat is at risk of tissue calcification, which
means that normally soft tissue in the body becomes hardened, which is very
painful and can adversely affect proper function. If calcification occurs
in the kidneys, kidney function is further worsened and a vicious circle
The test is run
five days a week (Mon-Fri), and turnaround time is 1-4 days. The sample needs to be sent
overnight as a chilled or frozen sample. There is more information
Your cat should
fast for eight hours before the test in order to avoid
may adversely affect the results. The blood can be taken by your own vet
(who will of course charge for his/her time and work), who can then send
the sample direct to MSU for analysis, using this
test form, which also includes guidelines
for how to take and send the sample.
As at April
2017, MSU charged US$40 to run both tests, but you can check the current
here (look under Endocrinology, no.
For people within the USA, MSU can also provide your vet with an insulated prepaid
UPS overnight mailing
envelope which costs US$19 using this
order form (no. 99220). You or your vet
will need to provide cold packs to place around the sample
(though I heard from one person who said
it was included in the package they received from MSU), but overall
this usually works out cheaper than paying for chilled overnight shipping
TREATING YOUR CAT WITHOUT VETERINARY ADVICE CAN BE
tried very hard to ensure that the information provided in this website is
accurate, but I am NOT a vet, just an ordinary person who has lived
through CKD with three cats. This website is for educational purposes
only, and is not intended to be used to diagnose or treat any cat. Before
trying any of the treatments described herein, you MUST consult a
qualified veterinarian and obtain professional advice on the correct
regimen for your cat and his or her particular requirements; and you
should only use any treatments described here with the full knowledge and
approval of your vet. No responsibility can be accepted.
If your cat
appears to be in pain or distress, do not waste time on the internet,
contact your vet immediately.
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