An underactive thyroid is common, and the NHS notes that its symptoms may be mild and may not be easy to notice. Tiredness, feeling cold, weight gain: each has a dozen ordinary explanations, and most people reach for those first. That is precisely why it goes undiagnosed, and why a blood test is the thing that settles it rather than how convincing the symptoms feel.

The good news is unusually clear-cut. It is managed with a tablet that replaces the hormone the body is not making, and with the right dose you should be able to live a normal life.

Key takeaways

  • Symptoms may be mild and easy to miss, which is the main reason it is diagnosed late.
  • It is diagnosed by blood test, measuring thyroid-stimulating hormone (TSH) and thyroxine (T4).
  • Treatment is usually levothyroxine, replacing the hormone the body cannot produce.
  • It is usually lifelong. You will normally take it for the rest of your life to stop symptoms returning.
  • The dose has to be right, and checked. Regular blood tests confirm it, and the wrong dose causes its own side effects.
  • Being on the wrong dose can cause feeling or being sick, diarrhoea, headaches or problems sleeping.
  • An overactive thyroid is the opposite problem, treated differently with medicines such as carbimazole.
  • Heat intolerance and sweating are relevant here, and levothyroxine can contribute to them.

What does an underactive thyroid feel like?

Mostly like being run down, which is why it is so often attributed to something else.

The NHS describes hypothyroidism as a common condition where the thyroid gland in your neck does not make enough hormones, causing symptoms like tiredness and weight gain, and states directly that symptoms may be mild and may not be easy to notice. Common symptoms include feeling extremely tired and feeling cold more than usual.

Feeling cold more than usual is a symptom that hides especially well here, where buildings are heavily air-conditioned and being cold indoors is entirely normal. Someone who is always reaching for a cardigan in an office is unlikely to think of their thyroid, and nobody around them will suggest it either.

Symptoms worth taking together rather than individually:

  • persistent tiredness that rest does not fix
  • feeling cold when others are comfortable
  • weight gain without a change in eating
  • dry skin and brittle hair or nails
  • constipation
  • low mood or difficulty concentrating
  • heavier or irregular periods

Any one of these on its own is unremarkable. Several together, especially persisting, is a reason to ask for a blood test rather than to keep explaining them away.

How is it diagnosed?

With a blood test, after an examination that may include your neck.

The NHS describes the process: a GP may examine your neck to find out whether your thyroid gland is swollen, and you may have blood tests to check how much thyroid-stimulating hormone (TSH) and thyroxine (T4) are in your blood. You may also be referred to a specialist for further tests.

The two measurements answer different questions. TSH is the signal the brain sends to the thyroid, and it rises when the thyroid is underperforming. T4 is the hormone the thyroid itself produces. Reading them together is what distinguishes an underactive thyroid from other causes of the same symptoms.

This is why self-diagnosis does not work here and why symptom checklists online are unhelpful in both directions. The symptoms are shared with anaemia, vitamin D deficiency, depression, sleep problems and simply being overworked. The blood test is what separates them.

What does treatment involve?

Levothyroxine tablets, usually for life, with regular blood tests to confirm the dose.

An underactive thyroid is usually treated with medicine that replaces the hormone your body cannot produce, and that medicine is usually levothyroxine tablets. The NHS is clear about duration: you will usually need to take them for the rest of your life to prevent symptoms returning, and you should be able to live a normal life.

The part that gets underestimated is dose monitoring. You will have regular blood tests to check you are on the right dose, because being on the wrong dose can cause side effects such as feeling or being sick, diarrhoea, headaches or problems sleeping. If you have side effects, that is a conversation with your GP or pharmacist rather than a reason to stop.

Two consequences follow for anyone who has moved countries, which describes a great many people here:

  • Continuity of monitoring matters. If your last thyroid blood test was in another country a while ago, arranging one locally is worth doing.
  • Consistency of product matters. Levothyroxine has a narrow margin between doses that work and doses that do not, so tell your prescriber if the brand or presentation you are given changes.

What about an overactive thyroid?

The opposite problem, with roughly opposite symptoms, and different treatment.

Hyperthyroidism means the gland produces too much hormone. Symptoms tend to be weight loss despite normal eating, feeling hot and sweating, a fast or irregular heartbeat, anxiety or irritability, tremor and difficulty sleeping.

It is treated with medicines that reduce hormone production, commonly carbimazole, and sometimes with other approaches depending on the cause. The catalogue also lists thiamazole, which is the same class.

Feeling hot and sweating heavily is another symptom that camouflages itself here, since so does everyone else in August. As with the underactive form, the distinguishing feature is the pattern taken together and the blood test that confirms it.

Does the climate interact with thyroid treatment?

Mainly through heat tolerance, and it cuts both ways.

NHS Specialist Pharmacy Service guidance notes that levothyroxine may contribute to excessive sweating and heat intolerance, and advises maintaining hydration, with consideration of reviewing thyroid control if symptoms suggest over-replacement due to dehydration.

In practice: if you are on levothyroxine and find summer notably harder to tolerate than the people around you, that is worth mentioning at your next review rather than accepting. It may be the climate. It may also be that your dose needs checking.

Storage applies as it does to everything else, and is covered in storing medicine in a Gulf summer.

Do and don’t

Do:

  • Ask for a blood test if several symptoms persist together.
  • Attend regular blood tests to confirm your dose.
  • Report side effects such as nausea, diarrhoea, headaches or sleep problems.
  • Tell your prescriber if your levothyroxine brand or presentation changes.
  • Arrange local monitoring if you moved countries while on treatment.

Don’t:

  • Don’t dismiss persistent tiredness as inevitable.
  • Don’t stop levothyroxine because you feel well; that is the treatment working.
  • Don’t adjust your own dose.
  • Don’t rely on an online symptom checklist in place of a blood test.

Frequently asked questions

Will I need to take levothyroxine forever? Usually yes. The NHS states you will normally need to take it for the rest of your life to prevent symptoms returning, and that you should be able to live a normal life on it.

Why do I need blood tests if I feel fine? Because the tests confirm the dose, and both under- and over-replacement cause problems. Feeling fine is the goal, and the tests are how it is kept that way as your requirement changes over time.

Could my tiredness be something else? Easily, which is the point of testing. Anaemia, vitamin D deficiency, sleep problems and depression produce overlapping symptoms, and the blood test is what distinguishes them.

Where to go next

The thyroid health category lists what is stocked here, and hypothyroidism and hyperthyroidism approach it from the diagnosis. The ingredients index maps active ingredients to brands, which matters for levothyroxine where consistency of product is worth attention.

MedicForce option: our thyroid listings show active ingredient and strength, which is the detail to match when continuing a prescription started elsewhere.