Short answer: Often, but it is a known interaction. Thiazide diuretics such as hydrochlorothiazide make the kidneys pass less calcium, and vitamin D helps the gut absorb more. The NIH Office of Dietary Supplements says the combination might lead to high blood calcium, especially in older adults and people with weaker kidneys or overactive parathyroid glands. Supplement amounts and blood tests are for the prescriber to set.
The hydrochlorothiazide label (Precautions) says thiazides "may decrease urinary calcium excretion" and "may cause intermittent and slight elevation of serum calcium" even without a calcium disorder. It adds that marked high calcium may point to hidden hyperparathyroidism, and that thiazides should be stopped before tests of parathyroid function.
Vitamin D works from the other side. ODS explains that vitamin D supplements increase how much calcium the intestine absorbs. Put together, less calcium leaves in urine while more comes in from food and supplements.
| Piece | Effect on calcium | Source |
|---|---|---|
| Thiazide diuretic | Less calcium lost in urine | Hydrochlorothiazide label |
| Vitamin D supplement | More calcium absorbed from the gut | ODS vitamin D fact sheet |
| Calcium supplement | More calcium taken in | Robien 2013 review |
ODS says the combination of thiazides with vitamin D supplements "might lead to hypercalcemia, especially among older adults and individuals with compromised renal function or hyperparathyroidism." A 2013 systematic review of drug and vitamin D interactions (Robien 2013) reached the same conclusion for thiazides combined with calcium and vitamin D supplements, while noting that most of the evidence came from case reports and small studies.
ODS lists the brand examples Hygroton, Lozol and Microzide for this group of diuretics. Calcium supplements are common among people taking bone medicines, so the overlap is not rare; see calcium and vitamin D with bone medicines.
ODS sets the tolerable upper intake level for vitamin D at 100 mcg (4,000 IU) a day for adults, and warns that vitamin D toxicity almost always comes from supplements and can cause kidney failure, soft tissue calcification, heart rhythm problems and death. It also says taking calcium supplements with vitamin D supplements may increase the risk of some side effects.
These limits are for the general population. For someone on a thiazide, the prescriber may want a lower amount or a calcium blood test, and the labels and ODS do not give a single safe dose for this combination.
Over-the-counter vitamins are easy to leave off a medicine list. Dose Track lets you add supplements alongside prescriptions, and the interaction checker flags known pairs, so the full list is in one place when you see the prescriber or pharmacist. Kidney stone questions with calcium are on calcium supplements and kidney stones.
Read more on Dose Track: Vitamin D2 vs vitamin D3 · Blood pressure · Interaction checker in Dose Track