13 Signs of An Underactive Thyroid: Do You Know What To Look For?

Also known as an underactive thyroid, hypothyroidism is caused by a defect in the thyroid gland that leads to low thyroid hormone levels.

Estimates suggest it affects up to 8.5% of people from Western countries, and around 20 million Americans (1).

What’s more worrying, however, is that up to 60% of people are undiagnosed, often because we don’t realise what the symptoms of an underactive thyroid are.

This is a researched list of the major signs and symptoms to look out for. If you have three or more symptoms, you should check your thyroid hormone levels at the doctor.

1. Excessive fatigue is the most common underactive thyroid symptom

Excessive fatigue is the most common underactive thyroid symptom

This symptom may seem very vague and general, but it’s not.

In the renowned DanThyr (Danish Thyroid) study from Denmark, 4 out of 5 men and women with hypothyroidism reported experiencing chronic fatigue. This is actually the number 1 most commonly reported symptom (2).

Excessive fatigue and weakness can be described as extreme lethargy, where you are feeling too weak to do your regular everyday activities. It’s also a common early sign of type 2 diabetes.

2. Weight gain or much greater difficulty losing weight

Weight gain or much greater difficulty losing weight with hypothyroidism

As the thyroid hormone regulates metabolism, an underactive thyroid often leads to a low metabolic rate.

The result is a tendency for body weight to increase during development of severe hypothyroidism. Likewise, this makes losing weight much more difficult.

In fact, a Danish study of newly diagnosed hypothyroidism patients found that patients weighed on average 7 kg (15.4 lbs) more than the control group (those with no thyroid disorder) (3).

But it turns out this weight gain is due in part to severe water retention caused by low thyroid hormone (4). Alongside a slower metabolism and low energy levels, these symptoms accumulate to drive thyroid-related weight gain.

3. You recently quit smoking

Ex-smokers have a greater risk of hypothyroidism

Weight gain is very common after an individual quits smoking.

A 10-year study found it causes an average of 4-5 kg gain (9-11 lbs), which is largely due to hormonal changes that occur after cessation (5).

One of the main hormonal changes is related to the thyroid. In fact, you have a 6-to 7-fold increase in the risk of developing hypothyroidism after quitting smoking (3).

For this reason anyone experiencing unwanted weight gain after quitting smoking should have thyroid function tested.

4. Memory loss and “Brain Fog” is a sign of hypothyroidism

Memory loss and “Brain Fog” can indicate a thyroid problem

The brain is a major target organ for thyroid hormones. This is why the onset of hypothyroidism (as an adult) can affect brain function.

Clinical trials and functional imaging studies of the brain confirm that advanced hypothyroidism is associated with cognitive decline- largely reversible with treatment (6).

In contrast, subclinical (early-stage) hypothyroidism is not linked with major neuropsychiatric problems, although studies show small (reversible) deficits in memory and executive function do occur. This is most often described by patients as experiencing “brain fog”.

5. Excessively dry skin

Does your skin frequently get dry and itchy? To the point where it’s very visible?

Excessively dry skin is actually the second most common sign of hypothyroidism (62% of patients) according to the DanThyr study (3).

Note that this symptom can overlap with early signs of insulin resistance.

6. High LDL cholesterol levels could be a warning sign

High LDL cholesterol levels could be a warning sign of hypothyroid

An increase in LDL particle number (or Apo-B levels) is a known risk factor for heart disease.

Thyroid hormones are known to affect the heart and blood vessels, and the weight of evidence suggests hypothyroidism (even in its early-stage) is associated with modest increases in LDL “bad” cholesterol, Apo-B levels and triglycerides (7, 8).

There are several proposed mechanisms behind this link, but just how influential hypothyroidism is on heart disease risk in the long-run is not well understood and potentially exaggerated (9). There are so many other lifestyle factors that influence these heart health markers.

For this reason I don’t consider high LDL on its own as a strong indicator of hypothyroidism.

7. Frequent muscle aches and tenderness

A common complaint of those with undiagnosed thyroid issues is frequent muscle aches.

It can be characterised by long-term and widespread pain and soreness. This pain is often not specific to one particular area- much like those diagnosed with fibromyalgia.

In fact, both an underactive thyroid and fibromyalgia can be linked to a food intolerance (quite likely FODMAPs).

8. Irregular menstrual periods could be an underactive thyroid symptom

Irregular menstrual periods could be a sign of hypothyroidism

Did you know women are thought to be 5 to 8 times more likely than men to have thyroid problems?

One well-known symptom is large changes to menstrual cycle length and blood flow, even Amenorrhoea.

Studies have found that 8-12% of women with healthy thyroid function experience irregular periods, compared to 23-68% of women with hypothyroidism (10).

9. Increased sensitivity to the cold

An underactive thyroid can can disrupt the body’s innate ability to regulate body temperature.

This may cause you to feel somewhat colder than usual, particularly in situations where others around you are not feeling it.

10. Neck discomfort or swelling

Neck discomfort or swelling is a sign of underactive thyroid

Just over 50% of patients in the DanThyr study reported that neck discomfort was a sign of hypothyroidism (3).

The discomfort can be caused by inflammation of the thyroid gland’s cells, known as thyroiditis. The gland can actually become smaller or larger (known as goitre), depending on the cause of the disorder.

Hashimoto’s thyroiditis is actually the most common cause of hypothyroidism, and is most typically described by patients as a sensation of “something in the throat.”

11. Constipation is a sign and symptom of hypothyroidism

Constipation is a sign and symptom of hypothyroidism

Constipation is a commonly reported sign, one that usually resolves after correcting a thyroid hormone imbalance.

The mechanism behind this link is unclear, though many propose it is because of the gut-thyroid connection.

Much like high cholesterol that was listed above, note that chronic constipation on its own is very unlikely to indicate hypothyroidism (11).

12. Feeling low or depressed

Feeling low or depressed

Mental health problems such as depression, chronic stress and anxiety tend to go hand-in-hand with hypothyroidism.

Whether this is a direct effect of hormonal imbalance is debatable though, as these symptoms are not experienced by all patients (12).

It seems more likely the result of numerous unmanaged hypothyroid symptoms that have accumulated over time to make life miserable.

13. Thinning hair

Hair loss is one of the most feared underactive thyroid symptom in women.

It occurs when the thyroid hormones are unbalanced, and can actually happen during both hypothyroidism (underactive) and hyperthyroidism (overactive).

In a study of 100 women experiencing hair loss, researchers estimated that hypothyroidism was responsible for 10% of participants (13).

Fortunately in the majority of cases hair grows back once the thyroid imbalance is treated.

Do you experience these underactive thyroid symptoms and signs?

If you have an underactive thyroid you may experience any number of these warning signs.

Generally speaking, problems tend to develop slowly over a number of years. It really depends on the severity of thyroid hormone deficiency and the length of time you’ve had it.

You should see your doctor to your TSH levels if you’re experiencing three or more of these signs, and be sure to get a copy of the results.



About Joe Leech, Dietitian (MSc Nutrition & Dietetics)

Joe Leech is a university-qualified dietitian from Australia.

He graduated with a Bachelor's degree in exercise science, followed by a Master's degree in Nutrition and Dietetics in 2011.

Learn more about him on the About page