My doctor also mentioned this today.

Weight loss can be a symptom of hyperparathyroidism, a condition that occurs when the parathyroid gland produces too much parathyroid hormone (PTH). Weight loss is usually a sign of an advanced stage of the disease.
Causes
Elevated calcium
High levels of calcium can affect the digestive system, causing nausea, vomiting, and constipation, which can lead to weight loss.
Metabolic effects
High levels of PTH and calcium can cause metabolic effects that lead to weight loss.
Adipose tissue browning
High levels of PTH can trigger a metabolic process called adipose tissue browning, which increases energy expenditure and can lead to weight loss.
Other symptoms
Other symptoms of hyperparathyroidism include: bone loss, kidney stones, and chronic fatigue.
Treatment
Surgery to remove a parathyroid tumor can help restore calcium levels and reduce the risk of weight gain.
Weight loss and other medical conditions
Weight loss can be a symptom of many different medical conditions, so seek medical advice if you experience sudden weight loss.