What could cause sudden weight loss in an apparently healthy toddler?

[Saruskabeth]

My son, 20 months old, suddenly dropped 4 lbs in 5 days. He has been taking longer, more frequent naps; at times it has been impossible to rouse him. (he fell asleep on the floor while playing by himself the other day and my husband shook him, rolled him over, even held him upside-down in an attempt to wake him up, but he couldn't keep his eyes open. He even stayed asleep during his diaper change!)  This is incredibly unusual, as he has always been difficult to put down for his nap or bed. When he's not falling asleep, though, he is running around, screaming and laughing, and seems to have 2x the energy. His appetite, too, has been going from one extreme to the next, from wanting to eat constantly to seeming totally uninterested in food, even sweets and (his absolute favorite) mashed potatoes. He has been thirstier than usual, even though the weather outside has been cooler and wet, not hot and dry. at around 12 months of age, his stools began to become more solid, but at around 16 months or so, they became softer, then very loose. The doctor said that as long as he has consistent stools it is not a concern, but I'm beginning to think it may be a symptom of something more severe. The weight loss, appetite, and sleepiness has all changed in the last two or three weeks. Am I overreacting, or is my concern justified?

[Deanna L Aftab Guy, MD]

Weight loss in a toddler can be due to a few endocrine problems, the biggest and most urgent is diabetes, if he really has lost 4 lbs, this is more than 10% of his body weight this is severe, have your doc see him face to face, check a urine, labs and then if all normal monitor him, get the fluids in him and be sure he is doing well.
Other hormone problems include hyperthyroidism but these kids do not have lethargy rather the reverse, weight loss, diarrhea and irritability, not what you are describing, then there is a rare form of diabetes called water diabetes, where they lose alot of free water in their urine and alot of weight and sodium rises.
I would have a cmp done, cbc, urine and see what it looks like.
Hope he is doing better! For all we know it could be tonsils, ears or urine infection, have him examined