What can cause very high blood calcium of 17 in dogs….

A calcium of 17 mg/dL in a dog is a medical emergency, and cancer is the most likely cause.

At that level, the main suspects are tumors that secrete parathyroid hormone-related protein (PTHrP), which mimics the body's calcium-raising hormone. In one case, a dog with malignant melanoma had calcium of 16.8 mg/dL and very high PTHrP [1]. In a larger study, dogs with lymphoma averaged 15.0 mg/dL, and those with anal sac adenocarcinoma averaged 15.0 mg/dL, with some reaching 18.4 mg/dL [2]. Parathyroid adenomas, benign tumors of the gland itself, averaged 16.1 mg/dL and reached 19.0 mg/dL [2].

Vitamin D poisoning is another real cause. Toxicity kicks in when 25(OH)D levels pass roughly 750 nmol/L, and it produces profound hypercalcemia [4]. Kidney failure and other cancers are also on the list, but at 17 mg/dL, a PTHrP-secreting tumor or a parathyroid adenoma is the leading suspect [2][3].

The retrieval here is thin on treatment and long-term outcomes. It does not cover modern protocols or survival statistics. What it does show is that the diagnosis hinges on measuring PTHrP, PTH, and vitamin D levels, plus imaging for tumors [1][2][4].

This is not a wait-and-see situation. At 17 mg/dL, the dog is at risk of kidney damage and heart rhythm problems. Get the blood work done now, and push for the tumor workup.

My call: cancer is the most probable cause, with vitamin D toxicity a strong second. Confidence: moderate, based on the case series and mechanism, but the retrieval lacks a modern differential breakdown.