This clinical case report describes the development of intoxication symptoms (nausea, muscle cramps, generalised weakness) and instability of blood pressure associated with long-term uncontrolled intake of high-dose vitamin D (up to 35,000 IU per week) in a 68-year-old female patient with comorbid conditions: secondary arterial hypertension, primary hyperparathyroidism, and chronic kidney disease stage C3a. Laboratory analysis revealed a typical triad of abnormalities: hypercalcemia (total calcium up to 2.82 mmol/L), elevated parathyroid hormone level (115.4 pg/mL), and a high serum 25(OH)D concentration (70.6 ng/mL). Key evidence of the iatrogenic nature of the symptoms was the complete resolution of complaints and normalisation of blood pressure on the previous antihypertensive therapy following discontinuation of vitamin D intake, as well as recurrence of symptoms upon resumption of vitamin D supplements, even at a lower dose (14,000 IU/day). A notable feature of this case is the development of toxic effects at a serum 25(OH)D level only slightly above the normal range, which underscores the importance of identifying adverse effects when prescribing therapy to patients with comorbidities. This observation highlights the necessity for thorough laboratory monitoring (calcium, parathyroid hormone, 25(OH)D, renal function parameters) prior to and during vitamin D supplementation, particularly in patients with hyperparathyroidism and chronic kidney disease.