What is the likely cause of weight loss and dyspnoea in a 55-year-old man who smokes and has lung cancer?

Hypercalcaemia associated with malignancy is a likely cause of weight loss and dyspnoea in a patient with lung cancer. When cancer spreads or is associated with certain paraneoplastic syndromes, it can lead to elevated calcium levels in the blood, a condition known as malignancy-associated hypercalcaemia. This occurs due to either osteolytic metastases or the production of parathyroid hormone-related peptide (PTHrP) which mimics the action of parathyroid hormone, leading to increased bone resorption and calcium release into the bloodstream.

The symptoms of hypercalcaemia can include significant weight loss and respiratory issues like dyspnoea. The increased calcium levels can result in various metabolic disturbances that contribute to weight loss, fatigue, weakness, and may even affect respiratory function due to muscle weakness or respiratory depression.

In this context, while malnutrition could contribute to weight loss, it does not directly explain dyspnoea. Increased metabolic demand could also contribute, particularly in the presence of cancer, but hypercalcaemia more directly correlates with the symptoms described. Sepsis, while a possible complication in cancer patients, tends to manifest with systemic signs such as fever and increased white blood cell counts, which are not