Can a blood test explain weight gain?

The question when the scale moves

The weight climbs a few kilos a year with nothing obvious having changed. The food looks the same, the exercise looks the same, and the number sits higher anyway. Then the question comes: is something going on in the body?

It is a reasonable question, and a blood test answers part of it. There is one common hormonal cause and one rare one, and both can be measured. Past those two, measurability runs out fairly quickly.

This article covers what a test can show for weight gain, how the proportions sit, and what remains when the results come back normal.

The thyroid

An underactive thyroid is the explanation people look for most often, and the only common one a blood test catches.

1177 describes the symptom picture in two stages. Early on you become more and more tired, you feel cold easily and your mood drops. Later you get dry skin and dry hair, you may lose hair, you gain weight and your pulse slows. Weight gain is on the list, but it rarely stands alone. If the weight arrives without tiredness, cold sensitivity and the other signs, the thyroid becomes a less likely explanation.

The condition is not rare. Internetmedicin states that primary hypothyroidism affects 2 to 3 percent of all women. 1177 writes that it is common and that many people have it for life.

The diagnosis is made with blood tests measuring thyroid hormone levels and TSH. A mildly raised TSH together with unspecific symptoms is not enough for a verdict. Internetmedicin says the sample should in that case be repeated after 6 to 12 weeks, with TPO antibodies added, before any conclusion. Treatment is medication, primarily levothyroxine, and most people need it for the rest of their lives.

Checking the thyroid is routine here. 1177 writes that in an obesity work-up the doctor measures blood pressure, takes blood tests including blood sugar and blood lipids, and checks how the thyroid is working.

Blood sugar and HbA1c

HbA1c shows, according to 1177, what your blood sugar level has been over the past two to three months. The test is sometimes called långtidssocker (long-term sugar). It can be taken at any time of day, and you do not need to prepare in any particular way.

Blood sugar is part of the obesity work-up at 1177, alongside blood pressure and blood lipids. The reason is that weight and blood sugar move together over time, and rising blood sugar is something healthcare wants to catch early.

This is where the documented ground ends. We found no Swedish authority source describing insulin resistance as an explanation for why weight has increased, in that direction. We therefore do not present an insulin value as an answer to that question. HbA1c reports how your blood sugar has run over recent months, and that is the question the value answers.

Cortisol and Cushing's syndrome

Cortisol comes up often in conversations about weight. There is a real disease behind it, and it is rare.

Internetmedicin puts the incidence of Cushing's syndrome at around 2 to 4 per million inhabitants per year. The symptom picture includes overweight and obesity tied to increased fat accumulation on the abdomen, above the collarbones and behind the eyes. The most common cause of clinical cortisol excess, according to internetmedicin, is treatment with cortisone preparations, meaning medication rather than adrenal disease.

How cortisol excess is investigated matters. Internetmedicin states that the recommended screening tests are cortisol in a 24-hour urine collection, an overnight dexamethasone suppression test, or salivary cortisol at 22 to 23 in the evening. A single cortisol sample taken in the middle of the day is not on that list. Cortisol follows the daily rhythm, and the tests are built to capture that rhythm.

1177 separately describes the body producing more cortisol under stress, and notes that cortisol can raise the risk of obesity and other illnesses. That is a different phenomenon from Cushing's syndrome, and it is not investigated with the same tests.

Energy balance

1177 states the basics without hedging. The energy your body needs to function comes from what you eat and drink. The body needs to use as much energy as it takes in. You gain weight if you take in more than you use.

That does not make willpower the whole story. 1177 writes that people with obesity have usually inherited many different genes affecting appetite and the body's ability to store energy as fat. Lifestyle habits affect weight, for example the kind of food you eat. Sleeping too little raises the risk of gaining weight. Under stress the body produces more cortisol, which can raise the risk of obesity. Certain medicines can contribute to weight gain. And overweight can sometimes stem from hormonal illnesses that affect appetite and metabolism.

That last line is where the blood test has its place. The test can show whether a hormonal cause is in the picture. Heredity, sleep, stress, medication and what you actually eat are not measured in blood, and they make up most of the list.

What a blood test settles

A blood test for weight gain does one bounded thing. It can show whether the thyroid is running slow, how blood sugar has tracked over recent months, and how blood lipids look. Where cortisol excess is suspected, specific tests exist and they are carried out within healthcare.

It cannot weigh in sleep, stress, medication, heredity or diet. It does not make a diagnosis on its own, and it says nothing about what you should weigh.

A normal result is also an answer. It means the measurable hormonal explanations are settled and the conversation moves to habits, sleep, medication and the support healthcare offers to people who want to change their weight. That is a less dramatic message than a diagnosis, and for most people it is the accurate one.

This article is general health information, not medical advice and not an assessment of your values. If you have symptoms or concerns, contact your healthcare provider.