Slow Metabolism Symptoms: The Signs I Look For, and What the Studies Say

The usual signs of a slow metabolism are cold hands and feet, constipation, tiredness, dry skin, low appetite in the morning and a low body temperature.

Almost all of them are also the classic signs of low thyroid function. That is not a coincidence. Thyroid hormone sets the rate at which your cells burn fuel.

What happens if your metabolism is slow?

Your metabolic rate is how fast your cells turn food and oxygen into energy and heat. When it drops, everything that depends on that energy slows down with it.

• You run cold. Less fuel burned means less heat. The hands and feet go first.
• Your gut slows down. Food sits longer, and you go less often.
• You are tired and flat. The brain is the first organ to notice a fuel shortage.
• Your skin gets dry and your hair thins.
• You are not hungry in the morning. In my view that is one of the clearest signs. Healthy children wake up hungry.

Doctors have tested how well symptoms like these predict thyroid disease. In 76 newly diagnosed hypothyroid patients and 147 matched controls, people who reported a change in 7 or more symptoms over the past year were far more likely to be hypothyroid, with a likelihood ratio of 8.7. People with 2 or fewer were less likely, at 0.5. (Canaris 1997)

So one symptom tells you little. A cluster that appeared recently tells you a lot.

Slow metabolism and thyroid

Low thyroid function is common. At a health fair in Colorado, 25,862 people had their TSH measured. It was elevated in 9.5% of them. Among people already on thyroid medication, 40% had an abnormal TSH. (Canaris 2000)

The same study found that hypothyroid people reported more symptoms, but that each single symptom was not very sensitive.

If you have several of these signs, get a blood test. Ask for TSH, free T4 and free T3. A thyroid problem needs a doctor, and thyroid hormone is a prescription drug.

Slow metabolism and constipation

This link has been measured directly. Researchers timed how long a test sugar took to reach the colon. It took 29 minutes in 10 hyperthyroid patients, 72 minutes in 42 healthy controls, and 80 minutes in 6 patients once they had become hypothyroid. When hypothyroid patients were given thyroid replacement, transit sped up significantly. (Shafer 1984)

My own rule is blunt. If you are not going two or three times a day, your metabolism is low. Food that sits gets fermented, and that is where the bloating comes from. That is my view, not a clinical cutoff. The gut side is in how to fix leaky gut.

Cold hands and feet, temperature and pulse

A warm body needs a fast metabolism. In my experience a healthy thyroid shows up first as warm hands and feet.

Ray Peat judged metabolic rate with a thermometer and a watch. He took the method from the doctor Broda Barnes, who used the temperature on waking, and he added the resting pulse because either number alone can mislead. He measured on waking and again after breakfast. In a healthy person both go up after eating. In a low thyroid person, stress hormones can hold the morning numbers up, and they fall once food brings the stress hormones down. He also noted that in cool weather the hands and feet can tell you more than the thermometer in your mouth.

The details, with the articles they come from, are in Ray Peat on temperature and pulse.

What slows it down

• Eating too little for too long. 24 people who had practiced calorie restriction for an average of 6 years, eating about 1,769 kcal a day, had a significantly lower 24-hour core body temperature than matched people on a normal diet. (Soare 2011)
• Crash dieting. 14 contestants from "The Biggest Loser" were measured 6 years after the show. Their resting metabolic rate was still 704 kcal a day below where it started, and about 500 kcal a day lower than their body size predicted. (Fothergill 2016)
• Cutting carbohydrates. Seven healthy young adults ate low-carbohydrate diets for a week each. T3, the active thyroid hormone, fell on both versions, and it fell more on the high-fat one. (Ullrich 1985)

How to fix a slow metabolism

The Vermont overfeeding studies are the mirror image of the dieting studies. When volunteers were overfed for 3 weeks, their production of T3 rose from about 30 to about 54 micrograms a day. In the long-term phases, T3 rose when carbohydrate replaced fat at the same calories. (Danforth 1979)

That does not mean overeating is the cure. It means the body turns thyroid up when fuel is plentiful and down when it is scarce. This is what I do with that:

1. Eat enough, and eat breakfast. No fasting.
2. Make carbohydrate the main fuel. Fruit, juice, milk, honey and potatoes.
3. Swap seed oils for butter and coconut oil.
4. Salt your food and get enough calcium and magnesium.
5. Get daylight and sleep.
6. Do not crash diet. Lose fat slowly.
7. Measure. Temperature and pulse, on waking and after breakfast, for a week or two.

The food side is laid out in the pro-metabolic diet.

The honest part

• "Slow metabolism" is not a medical diagnosis. Fatigue, feeling cold and constipation also come from anemia, depression, poor sleep, medication and many other things. See a doctor before you decide it is your thyroid.
• Treating a mildly abnormal lab value does not always help. In a trial of 737 adults over 65 with subclinical hypothyroidism, a year of levothyroxine did not improve the symptom score or the tiredness score compared with placebo. (Stott 2017)
• One of my most shared posts says that peeing too often is a sign of a low metabolic rate. The one study I found on this says otherwise. In 61 patients with thyroid disease, the hypothyroid patients urinated less often than they did after treatment. It was the hyperthyroid patients who went more often and at night. (Andersen 1987) Frequent urination has many causes, and it is a reason to see a doctor.
• The calorie restriction researchers think a lower body temperature may slow aging. I read the same data the opposite way. You should know that the mainstream view exists.
• In the Biggest Loser study, the people who kept the most weight off had the most metabolic slowing. A lower metabolic rate was the price of a large weight loss, not a sign they did something wrong.

Start with the thermometer and a blood test. Then fix the food. The reasoning behind the diet is in Ray Peat on thyroid.

Quick answers

What are the symptoms of a slow metabolism?

Cold hands and feet, a low body temperature, constipation, tiredness, dry skin, thinning hair and low appetite in the morning. These overlap with the symptoms of low thyroid function, and a cluster of several is more meaningful than any one alone.

What happens if your metabolism is slow?

Your cells produce less energy and heat. You feel cold and tired, digestion slows, and food stays in the gut longer.

Does a slow metabolism cause constipation?

Low thyroid function slows the gut. In one study, transit to the colon took 29 minutes in hyperthyroid patients, 72 minutes in healthy controls and 80 minutes in hypothyroid patients, and it sped up with thyroid replacement.

Is a slow metabolism caused by the thyroid?

Often. Thyroid hormone sets the metabolic rate, and in a survey of 25,862 people in Colorado, 9.5% had an elevated TSH. A blood test is the way to check.

How do you fix a slow metabolism?

Rule out thyroid disease with a doctor, eat enough, avoid crash diets and long fasts, and keep carbohydrates in the diet. Studies show that calorie restriction and low-carbohydrate diets lower body temperature or T3, and that overfeeding raises T3.

Are cold hands and feet a sign of a slow metabolism?

They can be. Less energy production means less heat, and the extremities cool first. Cold hands also have other causes, such as circulation problems, so persistent symptoms should be checked.