Can Thyroid Issues Impact Weight Loss?

Struggling to lose weight even though you’re doing everything right with your keto diet? It could be your thyroid! Thyroid hormones have a powerful influence over your metabolic rate, which means they can affect how readily your body will let go of excess fat. Let’s take a closer look at the effect of thyroid hormones on weight loss so you can determine whether low thyroid is the culprit behind your slow or stalled weight loss. 

Keep reading even if your doctor has told you for years that your thyroid is normal, or if you’ve already been diagnosed with low thyroid and you’re taking medication for it. There’s more you need to know!

How thyroid hormones impact body weight

Thyroid hormone is often called the “master regulator” of the metabolic rate. If you’ve noticed that some people seem to have a “fast metabolism” while others have a “slow metabolism,” thyroid hormones are major factors in this.

Signs and symptoms of low thyroid

How can you tell if you have low thyroid (a.k.a. hypothyroidism)? Here are the most common signs and symptoms:

• Brain fog
• Elevated cholesterol
• Fatigue
• Weight gain or difficulty losing weight
• Constipation
• Hair loss
• Low libido
• Slow heart rate
• Low blood pressure
• Depression, apathy, loss of motivation or ambition
• Low body temperature (often feeling cold)
• Edema (fluid retention, especially in the fingers and lower legs)

The classic and most common signs and symptoms of hypothyroidism all result from having a slower metabolism and lower-than-normal body temperature. In fact, before advanced testing was available, doctors diagnosed hypothyroidism based on body temperature and symptoms.

Every enzyme in your body – the molecules that control the rate of biochemical reactions – functions best at a specific temperature. If your body temperature is lower than normal because of hypothyroidism, these enzymes won’t function optimally, and this can compromise processes in every part of your body, including your brain.

Beyond symptoms, lab tests can either confirm that you do have low thyroid, or they can rule it out, in which case you can explore other possible causes for difficult weight loss. But in order to understand the problems with conventional thyroid tests, first you need to know a bit about the different kinds of thyroid hormones. 

What are the different thyroid hormones and what do they do?

If you’ve been struggling to lose weight despite strictly following a low-carb or ketogenic diet, you might already suspect your thyroid could be at fault.

But what if you’ve brought this up with your doctor and after some testing, they told you your thyroid is normal? Or maybe they diagnosed you with hypothyroidism and prescribed medication for you, but weight loss still feels impossible and you’re also experiencing some of the other signs and symptoms of low thyroid?

The issue might be that they did only limited testing. The “gold standard” thyroid test in conventional medicine is the TSH test – thyroid-stimulating hormone. Unfortunately, the TSH test does not always correctly identify hypothyroidism. It’s not uncommon to have a normal TSH level yet still be plagued by symptoms of low thyroid.

Here’s what you need to know about the key hormones related to thyroid function:

• Thyroid stimulating hormone (TSH): TSH is produced in the pituitary gland and it does just what its name says: it stimulates your thyroid gland to produce thyroid hormones. Elevated TSH is used to diagnose hypothyroidism. (A high TSH indicates low levels of circulating thyroid hormone.) However, not everyone with hypothyroidism has an elevated TSH. TSH may be normal in someone with low thyroid, and in rare cases, it can even be low. So if your doctor has tested only your TSH and it was normal, that doesn’t rule out the possibility that you have low thyroid.

• Thyroxine (T4): This is the primary hormone produced by the thyroid gland. T4 is considered an inactive hormone. It has little to no known biological activity and serves as a precursor to T3.

• Triiodothyronine (T3): T3 is the active form of thyroid hormone – the one most responsible for maintaining a normal metabolic rate and facilitating healthy cognitive function, a positive mental outlook, regular bowel movements, and all the other functions associated with proper thyroid hormone levels. The thyroid gland secretes just small amounts of T3. Most of your body’s T3 comes from T4 being converted to T3 in tissues other than your thyroid gland.

Many doctors don’t measure T3. They measure only TSH, based on this line of thinking: a normal TSH level indicates that your T3 level is normal. It’s assumed that if your T3 were low, it would signal your pituitary gland to pump out more TSH (in order to stimulate more T4 and T3 production), resulting in an elevated TSH. So if your TSH is not elevated, the logical conclusion is that your body has enough thyroid hormone and there’s no problem.

Unfortunately, it’s not this straightforward. It’s well known that the TSH test is not adequate for identifying all cases of hypothyroidism. If your TSH is normal but your T3 is low, you can see how this could result in you having a slower metabolic rate and struggling with weight loss, and likely also experiencing some of the other hallmark symptoms. But you won’t know your T3 is low if your doctor measures only TSH.

If you strongly suspect that you have hypothyroidism but you’ve been told for years that your thyroid is normal, request a comprehensive thyroid panel. This goes beyond just TSH and would include total or free T4, total and free T3, and reverse T3. (If you want to rule out an autoimmune issue, then include thyroid antibodies, too.)

What are the normal levels for thyroid hormones?

The most important thing when it comes to thyroid hormones is how you feel. “Normal” does not always mean optimal. Falling within the reference ranges doesn’t automatically mean you’ll feel your best.

Use your lab tests as a guide, but you are a human being, not a bunch of numbers on a page. More than your test results, how you feel – your signs and symptoms – should guide you and your doctor toward an appropriate course of action.

Far too many people with obvious signs and symptoms of suboptimal thyroid hormone levels are told everything is “normal” and they’re provided with no explanation for why they feel awful and are unable to lose weight.

A client of mine once said, “Normal is a setting on a washing machine!” I agree. “Normal” is not a useful adjective to describe hormone levels. I’ll say this again for emphasis: normal does not mean optimal. So if your doctor understands your concerns about the shortcomings of the TSH test and she orders a comprehensive thyroid panel for you but everything comes back “normal,” this still doesn’t rule out you having hypothyroidism.

Advocate for yourself. If your doctor insists that your thyroid isn’t the issue, then ask for their assistance in helping you explore other potential causes for the symptoms you are dealing with.

Problems with thyroid medication

It’s important to know that the most widely prescribed medication for hypothyroidism (levothyroxine or brand name Synthroid) provides only T4, with no T3. This works just fine for some people – it restores their hormone levels to normal, their symptoms are resolved, and they’re able to lose weight. However, there are many people for whom levothyroxine is not effective. 

Natural desiccated thyroid (NDT) is made from animal thyroid glands and it provides both T4 and T3. (The most common brand of NDT in the US is Armour. NP Thyroid is another popular brand.) A medication called liothyronine provides synthetic T3 without any T4. (The most common brand in the US is Cytomel.)

Unfortunately, doctors are often reluctant to prescribe medications that provide T3, but several international thyroid organizations have acknowledged that T4-only medications are not effective for some patients.

So, if you’re currently taking levothyroxine (Synthroid) but you still have unmistakable signs and symptoms of low thyroid and you can’t seem to lose weight even with a keto diet and regular exercise, ask your doctor about switching to an NDT medication or adding a T3 medicine to your current T4 regimen. 

Does Keto Cause Thyroid Problems?

There’s a lot of alarmism on social media about keto diets causing hypothyroidism. There is currently no evidence to support these claims. In an article about ketogenic diets and thyroid function, esteemed researcher Stephen Phinney, MD, PhD, wrote, “How many published studies from well designed, prospective human trials have shown that impaired thyroid function (i.e., hypothyroidism) occurs on a ketogenic diet? The answer to this question is quick and simple—NONE!”

Hypothyroidism can result from caloric restriction, but this is not unique to keto. This is a potential outcome with any eating plan on which someone chooses to overly restrict calories, and the risk for low thyroid might be greater when not eating enough is coupled with excessive exercise.

Some practitioners have begun referring to this as “famine-response hypothyroidism,” emphasizing that this is the body’s adaptation to perceived food shortage. Again, this is not a direct result of a keto diet.

Address other causes for slow weight loss

After carefully considering your symptoms and your lab tests – taking into consideration the shortcomings of the reference ranges – if you and your doctor definitively determine that you don’t have hypothyroidism, here are some other issues to address if you’re having a hard time losing weight on a keto diet:

• Watch your carb intake: The most important thing to do on a keto diet is to keep your carb intake ultra-low. If you’re currently counting net carbs, consider switching to counting total carbs. Eliminate questionable products like keto cereal, keto ice cream, and keto bread from your diet. These can be surprisingly high in carbs and aren’t as “keto” as you think they are.

• Don’t overdo fat: You can enjoy delicious, high-fat foods on a keto diet (ribeye, anyone?), but you can’t consume an unlimited amount of fat and expect your body to release a lot of its stored fat. Enjoy a sensible amount of fat with your food but don’t go out of your way to drown everything in a lake of melted butter or top every meal with a mountain of cheese.

• It could be your meds: Are you taking any medications that are known to cause weight gain or make it harder to lose? (See here for the Obesity Medicine Association’s list of meds known to cause weight gain.) Ask your doctor if there are any alternatives that will be effective for your condition without that side effect.

• Stress and sleep: Don’t underestimate the influence these lifestyle factors have on weight loss. If you’re stressed to the max and not sleeping enough (which itself is a stressor), you can’t blame your body if its top priority isn’t how you’ll look in a bathing suit this summer.

Summing Up

Thyroid hormones have a major impact on your metabolic rate, which in turn affects how readily your body will let go of excess stored fat. If you’re having a difficult time losing weight despite sticking to a strict keto diet and you’ve ruled out other causes for slow or stalled weight loss, investigate whether low thyroid might be the culprit, even if you’ve been told that your thyroid hormone levels are normal.

If you’d like to learn more about thyroid hormones, weight loss and keto, here’s a talk I gave about this topic at the Low Carb Denver conference in 2023: Optimizing Thyroid Hormones for Better Results on Low Carb Diets.

Looking for a delicious meal? Without the prep?

Then try Keto Chow! Keto Chow has 26 g. protein, 30+ vitamins and minerals, and zero added sugars. This meal shake comes in 25 delicious flavors, so you won’t hit that taste bud fatigue.