Underactive Thyroid Symptoms: Could Tiredness, Weight Gain and Feeling Cold Be a Sign?

Feeling tired, gaining weight without a clear reason, feeling colder than others, or struggling with brain fog can be frustrating. These symptoms are common, and they can happen for many reasons, including stress, poor sleep, iron deficiency, low mood, menopause, diet changes or other health conditions.

But they can also be signs of an underactive thyroid, also called hypothyroidism.

Your thyroid is a small gland in the front of your neck. It produces hormones that help control your metabolism, energy use, body temperature, heart rate, digestion and many other body functions. When the thyroid does not make enough thyroid hormone, several body systems can slow down.

Healthdirect Australia explains that hypothyroidism is when the thyroid gland does not make enough thyroid hormone, and symptoms can include tiredness, weight gain, bloating and sensitivity to cold. It also notes that the most common cause in Australia is an autoimmune condition called Hashimoto’s disease.

The challenge is that underactive thyroid symptoms can be slow and subtle. You may not suddenly feel unwell. Instead, you may gradually notice that your energy, weight, mood, skin, hair, periods or bowel habits have changed.

If these symptoms are persistent or affecting your daily life, it is worth speaking with a GP. A thyroid blood test can help check whether your thyroid hormone levels may be part of the problem.

What Is an Underactive Thyroid?

An underactive thyroid means your thyroid gland is not producing enough thyroid hormone for your body’s needs. The medical name for this is hypothyroidism.

Thyroid hormones affect how quickly or slowly many body processes work. When thyroid hormone levels are low, the body can slow down in different ways. This can lead to tiredness, weight changes, constipation, dry skin, low mood, slower heart rate, menstrual changes and feeling unusually cold.

Why the Thyroid Matters

Your thyroid helps regulate your metabolism. Metabolism is not only about weight. It also affects how your body uses energy, maintains temperature, supports digestion, regulates heart rhythm and responds to normal daily demands.

When thyroid hormones are too low, you may feel as if your body is running in “slow mode.” This can make normal activities feel harder than before, even if your routine has not changed.

For example, you may sleep enough but still wake up tired. You may eat similarly but notice gradual weight gain. You may feel cold when others feel comfortable. You may also notice constipation, dry skin, hair thinning or difficulty concentrating.

What Causes an Underactive Thyroid?

In Australia, one of the most common causes of hypothyroidism is Hashimoto’s disease, an autoimmune condition where the immune system attacks the thyroid over time. Healthdirect identifies Hashimoto’s disease as the most common cause of hypothyroidism in Australia.

Other possible causes can include:

• Previous thyroid surgery
• Radioactive iodine treatment
• Certain medicines
• Iodine-related issues
• Pituitary gland problems, although these are less common
• Previous thyroid inflammation
• Congenital thyroid conditions

A GP can help identify which cause is most likely based on your symptoms, medical history, examination and blood test results.

Can an Underactive Thyroid Develop Slowly?

Yes. Hypothyroidism often develops gradually. The Australian Thyroid Foundation describes hypothyroidism as usually gradually progressive, with symptoms increasing as thyroid function deteriorates.

This slow pattern is one reason people may not notice it at first. Symptoms may be blamed on ageing, work stress, parenting, sleep problems or lifestyle changes. While those things can contribute to fatigue and weight changes, persistent symptoms should still be checked if they are unexplained or getting worse.

Common Underactive Thyroid Symptoms

Underactive thyroid symptoms can vary from person to person. Some people have several symptoms, while others have only mild changes. Symptoms can also overlap with many other conditions, so thyroid disease should not be diagnosed based on symptoms alone.

A blood test is usually needed to check thyroid function.

The Australian Thyroid Foundation lists common hypothyroid symptoms such as lethargy or fatigue even after sleep, unexplained weight gain, low mood, brain fog, constipation, menstrual changes, cold sensitivity, thinning hair, brittle nails, dry skin, muscle pain and weakness.

Tiredness That Does Not Improve With Rest

Fatigue is one of the most common symptoms people associate with an underactive thyroid.

This is not always normal tiredness after a busy week. It may feel like heavy, ongoing tiredness that does not improve much with sleep. You may feel slower in the morning, struggle to concentrate during the day or feel unusually drained after normal activities.

However, tiredness is also one of the most common symptoms in general medicine. It can be linked with poor sleep, stress, anxiety, depression, low iron, vitamin deficiencies, diabetes, infections, sleep apnoea, menopause and many other causes. That is why a GP may check thyroid function alongside other possible causes.

Unexplained Weight Gain or Difficulty Losing Weight

An underactive thyroid can slow metabolism, which may contribute to gradual weight gain or make weight loss harder. Some people also experience fluid retention or bloating, which can make weight changes feel more noticeable.

However, thyroid-related weight gain is usually not the only explanation for major weight changes. Diet, activity levels, sleep, medicines, hormonal changes and other medical conditions can all contribute.

If your weight has changed without a clear reason, especially alongside tiredness, cold sensitivity, constipation or dry skin, thyroid testing may be worth discussing.

Feeling Cold When Others Feel Comfortable

Feeling unusually sensitive to cold is another common symptom of hypothyroidism.

You may notice that you need extra layers, feel cold in normal room temperatures, have cold hands or feet, or feel colder than people around you. This can happen because thyroid hormones help regulate heat production and energy use in the body.

Cold sensitivity can also happen with anaemia, circulation problems, low body weight or other conditions, so it should be assessed in context.

Constipation and Slower Digestion

When thyroid hormone levels are low, digestion may slow down. This can lead to constipation, bloating or feeling sluggish after meals.

If constipation is new, persistent or associated with other symptoms such as weight change, fatigue, abdominal pain or blood in the stool, it should be reviewed medically.

Dry Skin, Hair Thinning and Brittle Nails

Low thyroid hormone levels can affect skin, hair and nails. Some people notice dry, rough or flaky skin, hair thinning, hair shedding, brittle nails or thinning at the outer edges of the eyebrows.

These changes can also be caused by iron deficiency, stress, ageing, nutritional factors, skin conditions or hormonal changes, so a thyroid test may be part of a broader assessment.

Symptoms That Are Often Mistaken for Something Else

Underactive thyroid symptoms can be difficult to identify because they often look like everyday health problems. Many people feel tired, gain weight, feel low or notice changes in their skin and hair without immediately thinking about thyroid disease.

This is why hypothyroidism can be missed or delayed. The symptoms are real, but they are also non-specific, meaning they can happen in many different conditions.

For example, tiredness may be linked with poor sleep, stress, depression, iron deficiency, vitamin B12 deficiency, diabetes, infections, menopause or sleep apnoea. Weight gain may be linked with diet, activity, medicines, hormonal changes, fluid retention or other metabolic conditions.

This does not mean thyroid testing is always needed for every tired person. But if symptoms are persistent, unexplained or happening together, it is worth discussing with a GP.

Tiredness May Be Blamed on Stress or Busy Lifestyle

Many people first explain their fatigue as work pressure, parenting, poor sleep or stress. Sometimes that is correct. But thyroid-related tiredness can feel different because it may continue even when you are resting more.

You may notice that you are sleeping enough but still waking tired, needing naps, losing motivation or feeling slower during the day.

If fatigue continues for weeks or months, or appears with weight gain, cold sensitivity, constipation, dry skin or menstrual changes, thyroid function may be one of the things your GP considers.

Weight Gain May Be Blamed Only on Diet

Weight gain can happen for many reasons. But if your eating pattern and activity level have not changed much and your weight is still increasing, it is understandable to wonder whether something medical is involved.

An underactive thyroid can slow several body processes and may contribute to gradual weight gain. However, thyroid disease is not the only reason weight changes happen. Your GP may also consider blood sugar, cholesterol, medicines, sleep, menopause, mood, activity levels and other hormonal factors.

The useful question is not only, “Is my thyroid causing weight gain?” It is also, “What else in my health picture could explain this change?”

Low Mood or Brain Fog May Be Treated as Only Mental Stress

Low mood, poor concentration, forgetfulness and brain fog can happen with hypothyroidism. They can also happen with anxiety, depression, burnout, poor sleep, grief, low iron, vitamin deficiencies and many other issues.

If mental fog or low mood is new, persistent or happening with physical symptoms such as cold sensitivity, constipation, hair thinning or unexplained weight gain, a thyroid blood test may be part of a broader assessment.

Healthdirect lists poor memory and depression among symptoms that can occur with thyroid problems, alongside fatigue, cold intolerance, constipation, dry skin and weight gain.

Underactive Thyroid Symptoms in Women

Underactive thyroid can affect both men and women, but thyroid problems are commonly discussed in women because symptoms can overlap with menstrual changes, pregnancy-related changes, postpartum symptoms, perimenopause and menopause.

This overlap can make it harder to know what is causing the problem. A woman may notice tiredness, weight gain, irregular periods, heavy bleeding, mood changes or fertility concerns and assume it is simply hormonal or stress-related.

Sometimes it may be. But thyroid function is also worth considering when symptoms are persistent or unexplained.

Period Changes

An underactive thyroid may affect menstrual cycles. Some women may notice heavier periods, irregular bleeding, longer cycles or changes in cycle pattern.

These changes can also happen due to PCOS, fibroids, endometriosis, contraception changes, perimenopause, stress, weight changes or other health conditions.

If your period pattern changes and does not return to normal, it is worth speaking with a GP, especially if you also feel unusually tired, cold or notice unexplained weight gain.

Fertility and Pregnancy Planning

Thyroid hormones can play a role in reproductive health. If thyroid levels are not well controlled, they may affect ovulation, fertility, pregnancy planning and pregnancy monitoring.

If you are trying to conceive, pregnant, recently postpartum or have had previous thyroid issues, thyroid testing and medical review may be especially important.

The NIDDK lists heavy or irregular menstrual periods and fertility problems among possible symptoms of hypothyroidism.

Postpartum and Menopause Overlap

After pregnancy, many people experience tiredness, hair shedding, mood changes and weight changes. During perimenopause or menopause, changes in weight, mood, sleep, temperature regulation and periods can also occur.

Because these symptoms overlap with hypothyroidism, it can be difficult to know what is happening without testing. A GP may consider thyroid function along with iron levels, vitamin levels, menopause stage, mental health, sleep and other medical factors.

How is an Underactive Thyroid Diagnosed?

An underactive thyroid cannot be diagnosed from symptoms alone. A blood test is needed to check how your thyroid is working.

Your doctor may arrange thyroid function tests if your symptoms, history or examination suggest a thyroid problem. These tests commonly measure TSH, and may also measure free T4 and sometimes T3, depending on the situation.

Healthdirect explains that thyroid function blood tests are used to check levels of thyroid stimulating hormone, thyroxine and triiodothyronine, commonly known as TSH, T4 and T3.

What Is TSH?

TSH stands for thyroid stimulating hormone. It is made by the pituitary gland in the brain and acts like a signal to the thyroid.

When thyroid hormone levels are low, the brain usually sends more TSH to tell the thyroid to work harder. This means a high TSH can be a clue that the thyroid is underactive.

TSH is often the first test used when checking thyroid function.

What Are T4 and T3?

T4 and T3 are thyroid hormones. T4 is the main hormone made by the thyroid, and the body converts some T4 into T3, which is more active in tissues.

If TSH is abnormal, your doctor may check free T4 to understand whether thyroid hormone levels are actually low. RACGP guidance explains that if TSH is high, free T4 should be checked; elevated TSH with free T4 below the reference range diagnoses primary hypothyroidism.

Why One Result May Not Tell the Whole Story

Thyroid results need to be interpreted in context. A mildly abnormal result may need repeat testing, especially if symptoms are unclear or if you were recently unwell.

Some people have symptoms that feel like hypothyroidism but have thyroid results within the normal range. In that case, a GP may look for other causes of fatigue, weight change or cold sensitivity.

RACGP notes that symptoms compatible with hypothyroidism may occasionally persist even when TSH is within the normal range, which highlights why broader assessment is important.

How Is an Underactive Thyroid Treated?

An underactive thyroid is usually treated with thyroid hormone replacement medicine. The goal is to replace the thyroid hormone your body is not making enough of and bring your thyroid levels back into a healthy range.

The most common medicine used is levothyroxine, which is a synthetic version of thyroxine, also called T4. Healthdirect explains that hypothyroidism is often treated with thyroid hormone replacement therapy using a synthetic version of thyroxine.

Treatment should always be guided by a doctor because the right dose depends on your blood test results, age, weight, medical history, pregnancy status, other medicines and heart-health risk.

Levothyroxine Replaces Missing Thyroid Hormone

Levothyroxine does not “stimulate” the thyroid to work harder. Instead, it replaces the hormone that your thyroid is not producing enough of.

When the dose is right, many symptoms may gradually improve. Energy may become better, cold sensitivity may reduce, constipation may improve, and weight or fluid-related changes may become easier to manage.

However, symptoms do not usually disappear overnight. Thyroid hormone levels take time to stabilise, and your doctor may need to adjust the dose based on repeat blood tests.

The Dose May Need Adjustment Over Time

Finding the right thyroid dose can take time. If the dose is too low, symptoms may continue and TSH may remain raised. If the dose is too high, it can push the body toward an overactive thyroid state, which may cause symptoms such as palpitations, anxiety, sweating, shakiness or unexplained weight loss.

RACGP notes that if symptoms do not improve after starting levothyroxine, TSH should be remeasured. If TSH remains raised, the dose may need adjustment, while if TSH is within the reference range, another cause for symptoms should be considered.

This is why follow-up is important. The aim is not only to take medicine, but to make sure the dose is safe and effective for your body.

How You Take Thyroid Medicine Can Matter

Thyroid medicine can be affected by timing, food, supplements and other medicines. Some people have poor response to treatment because the medicine is not being absorbed properly or is interacting with another medication.

RACGP explains that poor response to thyroid replacement may be due to poor compliance, drug interactions or impaired absorption.

Your doctor or pharmacist may advise how and when to take levothyroxine, and whether to separate it from supplements such as iron or calcium. Do not change the way you take your medicine without checking, especially if your thyroid levels have been unstable.

Can an Underactive Thyroid Cause Weight Gain and High Cholesterol?

Yes, an underactive thyroid can contribute to weight gain and may also affect cholesterol levels. This is one reason thyroid testing may be considered when someone has unexplained weight changes, tiredness and abnormal cholesterol results.

When thyroid hormone levels are low, several body processes slow down. This can affect metabolism, fluid balance and how the body handles fats in the blood.

Thyroid-Related Weight Gain Is Usually Gradual

Hypothyroidism can make it easier to gain weight or harder to lose weight, but it is rarely the only explanation for major weight changes.

Some of the weight change may be linked to fluid retention, slower digestion, lower energy, reduced activity due to fatigue, or metabolic slowing. Diet, sleep, menopause, stress, medicines, activity levels and other health conditions may also contribute.

This is why a GP may review your thyroid results alongside other factors rather than assuming thyroid disease is the only cause.

Underactive Thyroid Can Affect Cholesterol

Low thyroid hormone levels can affect how the body processes cholesterol. Some people with hypothyroidism may have raised cholesterol, including LDL cholesterol.

This matters because high cholesterol can increase cardiovascular risk over time. If someone has high cholesterol along with tiredness, cold sensitivity, constipation, dry skin or unexplained weight gain, thyroid testing may be useful as part of the assessment.

Treating Thyroid Disease May Help, But It Is Not a Weight-Loss Treatment

If hypothyroidism is confirmed and treated properly, some thyroid-related symptoms may improve. But thyroid medicine should not be used as a weight-loss treatment in people who do not have hypothyroidism.

Taking more thyroid hormone than needed can be dangerous and may increase the risk of heart rhythm problems, bone loss and other complications.

The safest approach is to treat confirmed thyroid hormone deficiency, then review weight, cholesterol, blood pressure, diet, sleep and activity as part of the broader health plan.

When Should You Speak to a Doctor About Thyroid Symptoms?

You should consider speaking with a doctor if symptoms are persistent, unexplained or happening together. One symptom by itself may not point clearly to thyroid disease, but a pattern of symptoms can be worth investigating.

For example, tiredness alone is common. But tiredness with weight gain, cold sensitivity, constipation, dry skin, hair thinning, low mood and menstrual changes may make thyroid testing more relevant.

You should book a GP review if you have:

• Ongoing tiredness that does not improve with rest
• Unexplained weight gain or difficulty losing weight
• Feeling unusually cold compared with others
• Constipation that is new or persistent
• Dry skin, brittle nails or hair thinning
• Low mood, brain fog or poor concentration
• Heavy, irregular or changed periods
• Fertility concerns or pregnancy planning
• A family history of thyroid disease
• A visible swelling or lump in the neck
• High cholesterol without a clear explanation
• Previous thyroid disease or thyroid surgery
• Symptoms while already taking thyroid medicine

Healthdirect advises seeking urgent care for severe thyroid-related warning signs such as drowsiness that can lead to unconsciousness, slow breathing and low body temperature.

Do Not Self-Diagnose Based on Symptoms Alone

Many underactive thyroid symptoms overlap with other conditions. Fatigue, weight gain, low mood and hair changes can be linked with iron deficiency, vitamin B12 deficiency, diabetes, menopause, depression, sleep apnoea, medication effects and many other causes.

A thyroid blood test can help clarify whether thyroid hormone levels are involved. If thyroid results are normal, your GP can look for other possible explanations.

If You Are Already on Thyroid Medicine, Symptoms Still Need Review

If you take levothyroxine and still feel unwell, do not change your dose on your own. Your doctor may need to check TSH, free T4, timing of medicine, interactions, missed doses or other causes of symptoms.

Persistent symptoms do not always mean the thyroid dose is wrong. Sometimes another condition is contributing, and adjusting thyroid medicine without evidence can be unsafe.

How Panraa Clinics Can Help With Underactive Thyroid Symptoms

Underactive thyroid symptoms can be confusing because they often overlap with everyday health concerns. Tiredness, weight gain, dry skin, constipation, low mood, brain fog or feeling cold may be related to thyroid function, but they can also be caused by other conditions.

Panraa Clinics can help with non-urgent thyroid concerns such as:

• Discussing symptoms that may suggest an underactive thyroid
• Reviewing thyroid blood test results, including TSH, T4 and T3 where relevant
• Understanding whether symptoms may need further testing
• Discussing thyroid medication concerns or repeat prescriptions where clinically appropriate
• Reviewing symptoms that continue despite thyroid treatment
• Understanding whether other causes of fatigue, weight gain or low mood should be checked
• Planning follow-up testing if thyroid levels need monitoring

An Australian-registered doctor can help you understand whether your symptoms fit with thyroid disease, whether thyroid function testing may be needed, and what next steps are appropriate based on your health history.

Healthdirect explains that thyroid function blood tests check TSH, T4 and T3 levels and are used to assess thyroid problems.

Online care can be helpful for non-urgent thyroid symptoms and test-result reviews. However, urgent care is needed if you have severe drowsiness, confusion, very low body temperature, slow breathing, fainting, severe weakness or chest pain.

FAQs

What are the first signs of an underactive thyroid?

Early signs can be subtle and may include ongoing tiredness, feeling colder than usual, dry skin, constipation, low mood, brain fog, hair thinning, menstrual changes and gradual weight gain.

These symptoms can also happen for many other reasons, so a thyroid blood test is usually needed to confirm whether thyroid function is involved.

Can an underactive thyroid make you gain weight?

Yes, an underactive thyroid can contribute to gradual weight gain or make weight loss more difficult. This may happen because low thyroid hormone levels slow several body processes and can contribute to fluid retention, low energy and reduced activity.

However, thyroid problems are not the only cause of weight gain. Diet, activity, sleep, menopause, medicines, stress and other medical conditions can also play a role.

Why do I feel cold all the time?

Feeling cold all the time can be a sign of an underactive thyroid because thyroid hormones help regulate body temperature and energy use.

However, cold sensitivity can also be linked with anaemia, low iron, circulation problems, low body weight or other health conditions. If it is persistent or happening with fatigue, weight gain, constipation or dry skin, speak with a GP.

Can hypothyroidism cause low mood or brain fog?

Yes, hypothyroidism can contribute to low mood, poor concentration, slowed thinking and brain fog. Healthdirect lists poor memory and depression among symptoms that can occur with thyroid problems.

Because mood and concentration changes can also be caused by stress, depression, anxiety, poor sleep, low iron, vitamin deficiencies or other conditions, thyroid testing may be only one part of the assessment.

What blood test checks for an underactive thyroid?

A thyroid function test is used to assess thyroid activity. It usually checks TSH, and may also check T4 and T3 depending on the situation.

TSH is often the first marker doctors look at. If TSH is abnormal, your GP may review free T4 and other results to understand whether the thyroid is underactive. Healthdirect explains that thyroid function blood tests check TSH, T4 and T3 levels.

Can thyroid symptoms happen even if blood tests are normal?

Yes. Symptoms such as tiredness, weight gain, low mood and hair changes can happen even when thyroid blood tests are normal, because many other conditions can cause similar symptoms.

If thyroid results are normal, your GP may consider other causes such as iron deficiency, vitamin B12 deficiency, diabetes, menopause, depression, sleep apnoea, medication side effects or chronic stress.

Do I need thyroid medicine if my TSH is slightly high?

Not always. A slightly high TSH may need repeat testing, monitoring or further assessment before treatment is started. Treatment decisions depend on your TSH level, T4 result, symptoms, age, pregnancy status, heart risk, antibodies and overall health.

RACGP guidance notes that if symptoms do not improve after levothyroxine and TSH is within the reference range, another cause for symptoms should be considered.

Can I stop thyroid medicine if I feel better?

Do not stop thyroid medicine without speaking with your doctor. If you have confirmed hypothyroidism, stopping medicine may cause thyroid hormone levels to fall again and symptoms may return.

If you feel better or have side effects, your GP can review your blood tests and dose safely.

Conclusion

Underactive thyroid symptoms can be easy to dismiss because they often develop slowly and overlap with common issues such as stress, poor sleep, low iron, menopause or lifestyle changes. But if symptoms such as ongoing tiredness, unexplained weight gain, feeling cold, constipation, dry skin, hair thinning or brain fog are persistent, it is worth speaking with a GP.

A thyroid blood test can help check whether your thyroid hormone levels are contributing to your symptoms. If hypothyroidism is confirmed, treatment with thyroid hormone replacement can help restore hormone levels, but the dose needs proper medical monitoring.

If thyroid results are normal, your symptoms are still valid. Your GP can help look for other causes and guide the next step. The aim is not to self-diagnose, but to understand what your body is telling you and treat the right cause safely.