In short
- Honestly, up front: for most people blood tests do not explain weight gain. That does not mean measuring is pointless.
- A slow thyroid does cause weight gain, but usually only a few kilograms, mostly from water retention.
- Insulin resistance is more often a consequence of weight gain than its cause. But it changes what is worth doing about it.
- Cortisol is rarely to blame. Cushing's syndrome is rare and has typical signs. Measuring it in everyone only causes false alarms.
- Often overlooked: medicines. Some antidepressants, antipsychotics, corticosteroids and insulin raise weight.
When your weight climbs without an obvious change in what you eat, looking for a cause in the blood is understandable. This article tells you what is worth measuring and how much each cause explains. It also tells you where searching in the blood is a waste of time.
What blood can explain, and how much
Let's start with orders of magnitude, because that is what most texts on this topic are missing:
| Cause | How much it usually explains | What else you notice |
|---|---|---|
| Slow thyroid | A few kilograms, mostly water. | Feeling cold, dry skin, constipation, slow pulse, hair loss. |
| Insulin resistance | Not a direct cause, but it makes losing weight harder. | Waist size, high triglycerides, low HDL, darker skin in skin folds. |
| Polycystic ovary syndrome | Contributes through insulin resistance. | Irregular cycles, acne, excess hair growth. |
| Cushing's syndrome | Can be a lot, but it is rare. | Round face, fat on the trunk with thin arms and legs, purple stretch marks, bruising, weak muscles. |
| Low testosterone in men | A change in body composition, less muscle. | Lower sex drive, tiredness, less strength. |
| Medicines | Often a lot, and quickly. | Timing linked to starting the medicine. |
If you do not recognise your own picture in the table, the chance that blood will find a culprit is small. Measuring still makes sense — but as a starting point for action, not as a search for an excuse.
What is worth measuring
| Test | Why | For whom |
|---|---|---|
| TSH, plus fT4 and anti-TPO if it is abnormal | The only common hormonal cause worth ruling out in everyone. | Everyone with this question. |
| Glucose and HbA1c | Show whether metabolism is already affected. | Everyone. |
| Insulin with a HOMA-IR calculation | Estimates insulin resistance, which makes losing weight harder. | If your waist is above the limit or there is a family history. |
| Triglycerides and HDL | The ratio is a simple indicator of metabolic risk. | Everyone. |
| Ferritin with CRP and a complete blood count | Tiredness that leads to moving less often starts here. | If you are tired. |
| Morning testosterone with SHBG | In men it explains a change in body composition. | Men with additional signs. |
| Androgens and LH | When polycystic ovary syndrome is suspected. | Women with irregular cycles, acne, excess hair growth. |
Cortisol: when measuring it makes sense
"Cortisol from stress" is the best-selling explanation for weight gain. It is also the one that is most rarely true. Cushing's syndrome is a rare disease with a typical picture, not just increased weight. Look for a round face, fat on the trunk with thin limbs, wide purple stretch marks, easy bruising, and weak thigh and upper-arm muscles.
If you have these signs, testing is needed, but not with a single morning blood measurement. The right tests are late-night salivary cortisol, 24-hour urine cortisol or a suppression test. An endocrinologist guides the work-up.
Why a single blood cortisol measurement is misleading: cortisol changes through the day. The blood draw itself raises it in someone who is afraid of needles. In a person without the typical signs, the test causes more worry than benefit.
Medicines that raise weight
This is the most often overlooked and the easiest cause to check. If your weight started climbing in the weeks after starting a new medicine, a link is likely:
- Some antidepressants and mood medicines.
- Antipsychotics, markedly in some people.
- Corticosteroids with longer use.
- Insulin and some diabetes medicines.
- Beta blockers in some people.
- Hormonal contraception in a minority of women, usually modestly.
Never stop a medicine on your own. Tell your doctor what you notice. There is often another option within the same group.
What blood cannot see
- How much you eat. Everyone systematically underestimates their intake. Not out of dishonesty, but because estimating is hard.
- How much you move outside exercise. Energy use outside training quietly drops when you diet.
- Sleep. Chronically short sleep increases hunger and reduces fullness.
- Muscle loss with age. The same weight with less muscle means lower energy use.
- Alcohol. A regularly overlooked source of energy that also worsens sleep.
That is why the most useful report is the one followed by a re-check. Act for three months, then look again. If HbA1c, triglycerides and waist size have shifted, you are on the right track. That holds even when the weight moves more slowly than you would like.
Frequently asked questions
How many kilograms can a slow thyroid cause?
Usually a few kilograms, mostly from water retention. If your weight has gone up markedly, the thyroid alone generally does not explain it.
Is cortisol to blame for weight gain?
Rarely. Cushing's syndrome is a rare disease with typical signs: a round face, fat on the trunk with thin limbs, purple stretch marks, weak muscles. Without these signs, measuring cortisol usually only clouds the picture.
What is HOMA-IR?
A calculation from fasting glucose and insulin that estimates insulin resistance. It does not make a diagnosis, but it helps you understand why losing weight is harder and guides what to do.
Can I blame medicines for weight gain?
If your weight started climbing in the weeks after starting a new medicine, a link is likely. Do not stop the medicine on your own. Tell your doctor, as there is often another option.
Is it worth measuring if I know I do not move enough?
Yes, as a starting point. Values such as HbA1c, triglycerides and liver enzymes show whether metabolism is already affected. They also give you a measurable point to compare against in three months.
Sources: ATA/ETA — the effect of hypothyroidism on body weight · Endocrine Society — management of Cushing's syndrome and choice of tests · International PCOS guideline 2023 · EASO — management of obesity and the role of medicines that cause weight gain · EASD/ADA — insulin resistance and metabolic indicators.
