The tail is a savings account
A leopard gecko stores fat in its tail the way a camel stores it in a hump: a plump tail is money in the bank for lean days, illness or breeding. That design makes the tail an honest gauge. A tail that slowly thickens on good feeding and holds its width is a gecko in balance; a tail that keeps narrowing week by week is a gecko spending more than it banks, whatever the reason. Judge the trend against the neck: a healthy tail is at least as wide as the neck where it joins the body, and clearly fatter in the middle. One slightly slimmer week means little; a steady direction means something.
Spending more than it banks
Work through the outgoings. Floor temperature heads the list: leopard geckos digest using belly heat, and a warm hide floor sitting well below the high 80s to low 90s Fahrenheit slows digestion until meals deliver less than they should. A tankmate creates the second drain, because a bolder gecko eats first, claims the warm hide and stresses the quieter one off its food even without a single fight. Gut parasites take their cut invisibly, stealing nutrition from meals the gecko does eat. Females can draw the tail down during egg-laying seasons, and chronic gut infections, the serious end of the list, waste a gecko despite a full dish.
Auditing the intake side
Then check what is actually going in. Adults eat a few times a week, juveniles far more often, and a gecko offered food only occasionally cannot bank fat no matter how healthy it is. Look at the feeders themselves: appropriately sized insects, varied between types, gut-loaded and dusted, offered when the gecko is awake and hunting in the evening. Watch a full feeding rather than assuming; feeders that escape into the substrate, a gecko that strikes and misses repeatedly, or one that mouths food and drops it can each point to eyesight, mouth or coordination problems that a full dish alone would never reveal.
Weighing and watching at home
Start a weekly weigh-in on a small digital scale, in grams, at the same time of day, and take a photo from directly above once a fortnight; the pair catches change earlier than memory does. Note appetite honestly: how many feeders offered, how many taken. Read the droppings too, because loose, runny or foul droppings, or visible undigested insect parts, shift suspicion toward the gut. If the tail stabilises once temperatures and feeding are corrected, you have found your answer and the log proves it.
When thin becomes a vet visit
Book a reptile vet if the tail becomes slimmer than the neck, if spine or hip bones start to show, if the gecko turns lethargic, or if weight keeps falling despite corrected heat and generous feeding. Go sooner with runny droppings, a swollen belly or complete food refusal. Take a fresh droppings sample in a sealed pot, since parasite checks are a first-line test here and need material to work with. Arriving with dated weights and appetite notes in your gecko's Pet Passport on KinsLe lets the vet see the rate of loss immediately, and rate of loss is what decides how urgently they dig.