When a Teenage Girl Gains Weight Quickly, This Is What I Look For

By Tina Chagoury, Clinical Dietitian · 5 min read · 2026-09-28

Girls of fourteen, fifteen and sixteen arrive in my clinic having gained significant weight in a year or two. It is rarely just growing up.

Girls of fourteen, fifteen and sixteen come to my clinic having gained a significant amount of weight over the past year or two. Their periods are present, but something has changed, often the flow, often the regularity.

The response they usually get is that it is normal, that it is puberty, that it will settle. Sometimes that is true. Often it is not, and the years between fourteen and eighteen are exactly when intervening is most valuable.

What I look for

Insulin resistance

Adolescence involves a physiological rise in insulin resistance. That is normal and temporary.

What is not normal is when it is compounded by high refined carbohydrate intake, constant snacking, very little movement and poor sleep. Then it does not resolve as it should.

The signs are the same as in adults: constant hunger, afternoon sugar cravings, energy crashes after meals, fat gain concentrated around the middle, and in some cases darkened skin in the folds of the neck or underarms.

Early PCOS

If cycles are irregular, if there is acne that is not responding to the usual approaches, if there is unwanted hair growth or hair thinning, PCOS deserves consideration rather than dismissal.

PCOS diagnosed and supported at sixteen has a very different trajectory from PCOS discovered at thirty two after years of difficulty.

Thyroid function

Worth a full panel, not TSH alone, particularly where there is a family history.

Deficiencies

Iron and ferritin especially, given menstrual losses in a growing body. Also vitamin D and B12. These affect energy, mood and concentration, and they get attributed to being a teenager.

What is actually being eaten

Frequently the pattern is: nothing in the morning, very little during the day, then a large intake in the evening with a great deal of refined carbohydrate. That pattern alone produces weight gain, and it is entirely fixable.

Why testing early matters

Adolescents should be having annual checks from around fifteen or sixteen. Not because we expect to find disease, but because establishing a baseline and catching a pattern early is enormously more effective than discovering it a decade later.

The things I would want to see: fasting glucose and fasting insulin, HbA1c, a full thyroid panel, ferritin and iron studies, vitamin D, and a lipid panel.

What I would not do

I would not put a teenage girl on a restrictive diet. This is the age when a difficult relationship with food is established, and it lasts for decades. I see the results of it in women in their forties constantly.

I would not focus on weight. I would focus on structure: three real meals, protein at each, adequate sleep, movement she actually enjoys, and reduced grazing.

I would not dismiss her. The single most damaging thing that happens to girls at this age is being told that what they are noticing is not real.

The long view

What we are doing at this stage is not managing a number. It is setting a metabolic and behavioural foundation that determines how her thirties and forties go.

That is worth doing carefully, and it is worth doing without shame.

This article is nutrition education, not medical advice. Adolescent health concerns should be assessed by a doctor.

Tina Chagoury is a licensed clinical dietitian specialising in women's hormonal health across all life stages, and the creator of the Hormone Reset Method™. Learn more and start here.