24 hour urine test: Hold supplements?

Yes withhold calcium supplements for 48 hours before and during the test. During the test no supplements, caffeine, alcohol, nicotine. Some medication can alter the results, but only withhold medication with the prescribing physicians advice.
At least you don't have to worry about the higher calcium levels, until, unless you repeat the test.

Hello @dplx9467 - If you are comfortable sharing, were you given any resources or pamphlets before your test? Typically you are given instructions on what you can eat and drink before any test. Are you able to take the test again, or were you able to communicate that you were taking supplements while taking this test?

I was given instructions by my endocrinologist but the instructions did not include holding supplements or any dietary changes. I am going to repeat the test and have put in this question to my endocrinologist but I was interested in other's experience. The calcium in my urine was 450 but I have been taking calcium supplements for decades.

I would think one should continue with their regular intake to truly represent the calcium level they are experiencing on a daily basis.

There's a difference between urine calcium and serum calcium. The supplements will artificially raise urine calcium and skew the test. You want to see urine calcium based on the normal diet. If that is high, then you have an issue. From another site, this is a good explanation:

"The reason to stop calcium supplements before a 24-hour urine calcium test is that the supplements will increase urine calcium levels. A study titled 'Do calcium supplements increase serum and urine calcium levels in post-menopausal women?' obtained the following results with calcium plus vitamin D:

'The serum calcium levels did not vary significantly in SG (study group) in spite of consuming calcium supplements over a period of time, whereas the urinary calcium levels increased progressively (p value < 0.005) in those who have taken calcium supplements for a year compared to those who have taken for a month.'

https://link.springer.com/article/10.1007/s12603-014-0532-2#:~:text=relevant%20statistical%20methods.-,Results,have%20taken%20for%20a%20month. "

If I take a Ca supplement, 365 days a year, then that is my normal level. Withholding it takes me out of the normal and may provide misleading info. If I am overtaking the supplement, it is important to know that.

This is a great question.

when I got my 24hr urine calcium tested after op diagnosis, no instruction was given to withhold calcium intake - although I get my calcium from natural food sources only (no milk products such as almond/soy milk with added calcium or other calcium fortified food products). I ate as usual and results came back normal.

One of my relatives is going to get 24hr urine calcium tested soon to explore secondary causes of her osteopenia. She told me the fellow (not endo herself) answered without hesitation with a "no" to the question whether or not to withhold calcium intake. "Otherwise it defeats the purpose", the fellow said. My relative does get calcium from natural food sources and the fellow is aware of that. She fell short to ask about calcium supplements but I guess it's not relevant to her situation.

Thank you @njx58 for the link. Many online search results came back with answer same as @gently's. I read one AI overview which says "To prepare for a urine calcium test, you may need to stop taking calcium supplements for a period of time, depending on what your doctor is trying to determine".

I think it's very important to ask our doctors about this when urine calcium test is ordered. So many factors could affect urine calcium levels. A few posters had high urine calcium level during anabolic therapy (forteo or tymlos). It is important to differentiate whether hypercalciuria is a preexisting (could be pathological) condition which might need to be treated or a temporary one as a result of anabolic therapy. I had high blood calcium level around 10.4mg/dl for a couple of months during forteo therapy but urine calcium was never re-examined.

ps below is a list of factors causing high levels of urine calcium (above 300mg/day) listed on UCSF Health:
•Chronic kidney disease
•High vitamin D level
•Leaking of calcium from the kidneys into the urine, which may cause calcium kidney stones
•Sarcoidosis
•Taking too much calcium (usually antacid pills)
•Too much production of parathyroid hormone (PTH) by the parathyroid glands in the neck (hyperparathyroidism)
•Use of loop diuretics (most commonly furosemide, torsemide, or bumetanide)

njhornung, I see your point.
Suppose the use of the test isn't to discover the body's experience on a daily.
Generally the 24 hour urine is used to detect possible problems with the parathyroid gland or possible problems with the function of the kidneys.
An abnormal test result generates further testing.

I would definitely check with the doctor who ordered the test about what if anything to withhold.

For example, Tymlos can cause hypercalciuria, which may cause kidney stones. In that case the level including the effect of supplements is relevant.

Serum calcium test should go hand-in-hand with urine calcium test.