Slow metabolism: is it real, and is it your thyroid?

A slow metabolism is real, but almost never in the way people mean. People with obesity burn more calories at rest, not fewer. Your thyroid explains a few kilos at most, and largely fluid. Cushing's syndrome affects 1 to 2 people per million per year. The suspect that is actually common: your medicine cabinet.

This article works through the suspects in the order people name them. It ends with the honest reason to test, which is not the reason you came here with.

Does a slow metabolism really exist?

Differences exist, but they are small and they usually run the other way. Measure resting metabolic rate and it is higher, in absolute terms, in people with obesity than in lean people. There is simply more body to maintain. Adjusted for body composition, a genuinely slow metabolism is uncommon.

What does exist is adaptation after weight loss. Per kilo lost, energy expenditure falls by roughly 20 to 30 kcal per day. Appetite rises by roughly 100 kcal per day (PMID 29156185).

Hunger is therefore three to five times the size of the metabolic effect.

In The Biggest Loser participants, resting metabolic rate was still about 500 kcal per day below prediction six years later (PMID 27136388). That is real, but it is not permanent metabolic damage. And it explains nothing about weight you never lost in the first place.

Is the scale simply stuck? That is almost never your metabolism. We wrote that up separately in why your plateau is not a slow metabolism.

Is it your thyroid?

Almost never. Overt, treatable hypothyroidism causes a few kilos, and that is largely fluid rather than fat. Treating it produces only modest weight loss. Then the point you rarely hear: your TSH is often mildly raised because of the excess weight, and it settles again after you lose weight.

That is reverse causation. The weight pushes the value up, not the other way round.

One hard conclusion follows. Subclinical thyroid abnormalities should not be treated in order to lose weight. Levothyroxine is not a weight loss drug, and Dutch GP guidance from the NHG and Thuisarts is cautious about treating a mildly raised TSH without symptoms.

Do you have symptoms that fit the thyroid, such as feeling cold, constipation, a slow heart rate and dry skin? Raise it with your GP. The full explanation is in thyroid and weight.

The suspects, honestly ranked

Four of the five classic suspects almost never explain your weight. The fifth, medication, is more common than all the others combined and is almost never mentioned. This table puts numbers on each one, so you know what to expect from testing.

Is it cortisol or Cushing's syndrome?

Almost certainly not. Cushing's occurs in roughly 1 to 2 people per million per year. Testing everyone who gains weight mostly generates false positives, cost and anxiety. Investigation is worthwhile with specific signs, not with central weight gain and fatigue.

Specific signs include broad purple striae, thin skin that bruises easily, muscle weakness in the thighs and a face that has become round quickly.

The cortisol belly story from the wellness industry is much weaker than it sounds. Outside genuine Cushing's, cortisol does not explain your weight.

Is it insulin resistance?

Mostly the other way round. Insulin resistance is largely a consequence of fat mass, not its cause (PMID 28074888). Weight loss improves your insulin sensitivity, not the reverse. In the DiRECT trial, type 2 diabetes went into remission in a substantial share of participants after significant weight loss (PMID 29221645).

One more popular claim has been tested and failed. The idea: if you are insulin resistant, you should eat low carb.

DIETFITS tested exactly that. There was no diet by insulin secretion interaction, and no diet by genotype interaction (PMID 29466592). Low carb and low fat produced nearly identical results.

Want to know what insulin resistance actually is and how it is measured? Read recognising insulin resistance.

The cause almost nobody mentions: your medication

This is the suspect that often is right. In a systematic review, people on olanzapine gained 2.4 kg on average, and people on pioglitazone 2.6 kg (PMID 25590213). Several widely used drugs do something similar, and almost nobody mentions it.

Think of mirtazapine, paroxetine and amitriptyline. Think of lithium, valproate, gabapentin and pregabalin. Think of insulin, sulfonylureas and corticosteroids.

Never stop a medicine on your own.

What you do instead: raise it with your prescriber. For many of these drugs there is an alternative that is weight neutral, or that at least causes less gain. That conversation is often worth more than any hormone panel.

PCOS: the exception that is common enough

PCOS is the one condition on this list common enough to genuinely warrant attention. Roughly 10 to 13 percent of women have it. Irregular or absent periods, excess hair growth, acne and difficulty losing weight all fit the picture.

Does that sound like you? See your GP. This is one of the few situations where targeted hormonal testing genuinely pays off.

Can you speed up your metabolism?

Not meaningfully with food, supplements or green tea. Those effects are small, short lived and invisible on the scale. What you can raise is your total daily expenditure, and there are only two routes: build muscle and move more.

Be honest about the return. Exercise alone produces about 1.6 to 1.7 kg of weight loss over six to twelve months (PMID 21787904). Your body also compensates away roughly 28 percent of the calories you burn through activity (PMID 34453886).

That is not a reason to skip exercise.

It is a reason to train for your heart, your head and your muscles, and to steer your weight in the kitchen. How to combine the two is set out step by step in healthy weight loss: the complete plan.

What a blood test does and does not do for you

Honestly: a blood test will not tell you why the scale is stuck. For most people it comes back normal. Testing to find a hidden cause usually buys you either expensive reassurance or a false alarm.

There is a good reason to test. It is simply a different reason from the one most people arrive with.

A blood test shows what your weight is currently doing to your metabolic health: fasting glucose, HbA1c, lipids, liver values. And it lets you watch those improve as you lose weight. That is measurable progress that does not depend on what the scale decides that morning.

If you want to track that, the complete metabolic panel is built for exactly that. Not to find a culprit, but to see your progress. What changes along the way is covered in what weight loss does to your blood values.

When this article is not for you

Weight loss is not a healthy goal for everyone. Do not follow this article if you have or have had an eating disorder, are pregnant or breastfeeding, are under 18, are underweight, use insulin, or have had bariatric surgery. Speak to your GP first.

If thoughts about food or your body start to take over, talk to your GP or contact an eating disorder helpline. That is not weakness. That is sense.

What to do now

Pull up your medicine list today. If a drug on it is known for weight gain, put it on the agenda for your next appointment with your prescriber. Ask explicitly whether a weight neutral alternative exists.

If you have symptoms that fit the thyroid or PCOS, book an appointment with your GP. If you do not, your gains are not in a hormone panel. They are in protein, strength training, sleep and a deficit you can hold for months.