Feeling tired, gaining weight or feeling unusually cold? Understanding the signs of an underactive thyroid

You’ve been feeling tired for a while, and you're starting to wonder if it's more than just a busy schedule.

Maybe you're sleeping reasonably well but still waking up exhausted. Your weight has gradually increased even though you don't feel like you've changed your eating habits much. You find yourself reaching for another layer when everyone else seems comfortable, and your skin feels drier than usual.

None of these things automatically means you have a thyroid problem. They're common experiences that can have many explanations. But when several of them appear together and continue over time, it's reasonable to wonder whether your thyroid could be involved.

An underactive thyroid, also called hypothyroidism, happens when the thyroid gland doesn't produce enough thyroid hormones for the body's needs. These hormones influence how the body uses energy and affect many organs, so when hormone levels are too low, several body processes can gradually slow down. The NIDDK explains how hypothyroidism affects the body.

The difficulty is that hypothyroidism often develops slowly. You may not wake up one morning feeling dramatically different. Instead, small changes can accumulate until you begin to realise that you haven't felt quite like yourself for some time.

You feel tired even after you've rested

One of the most common things people notice is fatigue.

It may feel like your energy has gradually disappeared rather than suddenly stopping. Getting through a normal workday becomes harder, concentration takes more effort and activities that once felt routine leave you feeling drained. You may also notice that sleeping doesn't seem to restore your energy in the way it used to.

Fatigue has an enormous number of possible causes, including inadequate sleep, stress, anaemia, nutritional deficiencies, infections, medication effects and other health conditions. That is why tiredness on its own isn't a reliable way to identify hypothyroidism.

The clue becomes more meaningful when fatigue appears alongside other changes, particularly if those changes have developed gradually and don't have an obvious explanation.

You're gaining weight without an obvious change in routine

Perhaps you've noticed your clothes fitting differently over the past few months.

You're eating roughly the same foods, your activity hasn't changed dramatically, yet your weight seems to be creeping upward. It's tempting to immediately blame metabolism, especially when you've heard that an underactive thyroid can cause weight gain.

There is some truth behind that association. Weight gain is a recognised symptom of hypothyroidism, although the amount can vary and many other factors can affect body weight.

An underactive thyroid doesn't explain every unexplained change on the scale, and weight gain by itself isn't enough to diagnose a thyroid disorder. Your diet, activity, sleep, medications, hormones and other health conditions can all influence weight.

What matters more is whether the weight change is happening alongside a broader pattern of symptoms.

You suddenly seem much more sensitive to the cold

Maybe everyone else is comfortable, but you're reaching for a jacket.

The room doesn't seem particularly cold, yet your hands and feet feel chilly and you find yourself wanting an extra layer. You may notice this happening more often than it used to. Feeling cold can be one of the symptoms of hypothyroidism because lower thyroid hormone levels can slow some of the body's normal processes.

But again, context matters. Your environment, body composition, circulation, clothing and even the weather can influence how cold you feel.

It's the change from your usual experience that may be worth noticing, especially when it appears together with persistent fatigue, weight changes or other symptoms associated with an underactive thyroid.

Your skin and hair don't feel quite the same

You may notice that your skin has become unusually dry, or that your hair seems thinner than before.

Some people with hypothyroidism also experience dry or thinning hair, hair loss or changes in their skin. These changes can be frustrating because they often seem cosmetic at first, even when they're occurring alongside other changes in how you feel.

There are many other explanations, however. Seasonal dryness, nutritional deficiencies, stress, hormonal changes and dermatological conditions can all affect the skin and hair.

So rather than treating dry skin or hair loss as evidence of thyroid disease, look at whether these changes are part of a larger pattern.

Your digestion has slowed down

Your digestion can also change when thyroid function is low.

Constipation is a recognised symptom of hypothyroidism, and some people notice that their bowel movements have become less frequent or more difficult than before.

Constipation is extremely common and can happen because of low fibre intake, inadequate fluids, changes in activity, medications and many other causes. It isn't unusual to experience it occasionally.

If it becomes persistent and appears alongside other changes such as fatigue, feeling unusually cold or unexplained weight gain, however, it becomes another piece of information worth mentioning to a healthcare professional.

You're finding it harder to concentrate

You sit down to work, but your thoughts don't seem to move as quickly.

You reread an email. You lose track of what you were doing. You feel mentally slower or find it harder to concentrate than you normally would.

Some people with hypothyroidism experience difficulty concentrating or what is sometimes informally described as "brain fog." The NHS lists difficulty concentrating or thinking clearly among possible symptoms of an underactive thyroid.

But concentration can be affected by sleep deprivation, stress, anxiety, depression, medication, nutritional deficiencies and simply having too much going on at once.

A change that persists alongside several physical symptoms is more useful information than an occasional unfocused afternoon.

Your muscles or joints are starting to ache

Perhaps your body feels heavier than usual.

You wake up with muscle aches, your joints feel uncomfortable or ordinary physical activity seems to leave you more sore than it used to. Muscle and joint pain can occur with hypothyroidism, but these symptoms are also common in everyday life and across many different medical conditions.

The pattern matters. If aches are persistent and you cannot connect them to exercise, injury, posture or another obvious cause, mentioning them alongside your other symptoms can help a healthcare professional decide what needs to be investigated.

Your periods have become heavier or irregular

For people who menstruate, changes in the menstrual cycle can sometimes provide another clue.

Hypothyroidism can be associated with heavier or irregular periods and can also affect fertility. This doesn't mean that every change in your cycle is caused by your thyroid. Menstrual patterns can change for many reasons, including stress, pregnancy, perimenopause, PCOS and other hormonal conditions.

But if a noticeable change in your periods appears alongside fatigue, weight changes, feeling cold or other symptoms, it is worth including that information when discussing your health with a professional.

Could it be Hashimoto's disease?

One common cause of an underactive thyroid is Hashimoto's disease, an autoimmune condition in which the immune system attacks the thyroid.

Over time, this can reduce the thyroid's ability to produce enough hormones.

Hashimoto's is more common in women and can occur in families with a history of thyroid or autoimmune conditions. The NIDDK's information on Hashimoto's disease explains how the condition can lead to hypothyroidism.

There are other causes too, including thyroid inflammation, previous thyroid surgery or radiation treatment, certain medications and, less commonly in some populations, inadequate or excessive iodine.

Knowing the cause can matter because treatment and follow-up may depend on why the thyroid is underactive.

What if you have several symptoms but aren't sure?

This is where thyroid problems can become confusing.

You may have fatigue, weight gain and feel cold more often, but those symptoms don't tell you whether your thyroid is actually underactive. Someone with poor sleep could experience some of the same things. So could someone with anaemia, certain nutritional deficiencies or another hormonal condition.

That's why hypothyroidism isn't diagnosed from symptoms alone.

If your healthcare professional suspects a thyroid problem, they may order blood tests to assess thyroid function. The NIDDK's guide to thyroid tests explains that TSH is usually checked first, with additional tests such as T4 used to understand what is happening.

In many cases, blood testing provides a much clearer answer than trying to match your symptoms against an online checklist.

When should you get your thyroid checked?

You don't need to have every possible symptom before speaking with a healthcare professional.

If you've been persistently tired, have noticed unexplained weight changes, feel unusually cold or have developed several of the symptoms described above, it is reasonable to discuss them with your doctor.

It can also be particularly useful to mention a family history of thyroid disease or autoimmune conditions, a recent pregnancy, previous thyroid treatment or medications that could affect thyroid function.

The NHS recommends seeing a doctor if you think you may have an underactive thyroid because many of its symptoms are common and can have other causes.

What happens if the tests show an underactive thyroid?

If blood tests confirm hypothyroidism, treatment is usually straightforward.

The usual treatment is levothyroxine, a medicine that replaces the thyroid hormone your body is no longer producing in sufficient amounts. The dose is adjusted according to your individual needs, and blood tests are used to make sure the hormone levels are appropriately controlled.

It's important not to start, stop or change thyroid medication without medical guidance. Thyroid hormone affects many systems throughout the body, and taking too much can also cause problems.

For people who have confirmed hypothyroidism and receive appropriate treatment, symptoms can generally be managed effectively.

The important part is the pattern

Feeling tired doesn't necessarily mean your thyroid is slowing down.

Gaining a few kilograms doesn't automatically mean your metabolism is being affected by thyroid hormones. Feeling cold doesn't mean you have hypothyroidism either.

But if you've noticed several changes happening together , persistent fatigue, gradual weight gain, increased sensitivity to cold, dry skin, constipation, difficulty concentrating or muscle and joint discomfort, it's worth stepping back and looking at the whole picture.

Your body rarely gives you a single symptom that neatly identifies the cause. More often, it gives you a collection of small changes that become meaningful when you look at them together.

This is where keeping track of what you're experiencing can be useful. Rather than trying to remember when the tiredness started or whether your symptoms have actually changed over the past few months, recording them over time can give you a clearer picture to discuss with a healthcare professional.

Clyvera can help you organise symptoms and health information over time, making it easier to recognise patterns, prepare questions and have a more informed conversation about what you're experiencing.

The goal isn't to diagnose yourself from a list of symptoms.

It's to notice when your normal has changed, and know when that change is worth investigating.