What is hypothyroidism?
The thyroid is a small, butterfly-shaped gland in your neck that produces hormones controlling how your body uses energy — your metabolism. In hypothyroidism, the thyroid does not make enough of these hormones, so many body processes slow down. This affects nearly every part of the body, which is why the symptoms can be wide-ranging and sometimes vague.
Hypothyroidism is common, especially in women and with increasing age. The good news is that it is straightforward to diagnose with a blood test and very treatable with daily hormone replacement.
Symptoms of an underactive thyroid
Symptoms develop slowly, often over months or years, and can be easy to attribute to other things. Common symptoms include:
- •Tiredness and a lack of energy
- •Weight gain
- •Feeling cold easily
- •Dry skin and hair, or hair thinning
- •Constipation
- •Low mood or depression
- •Slowed thinking, poor memory or 'brain fog'
- •Muscle aches and weakness
- •Heavier or irregular menstrual periods
What causes it?
The most common cause worldwide of an underactive thyroid in areas with enough dietary iodine is an autoimmune condition called Hashimoto's thyroiditis, in which the immune system attacks the thyroid gland. Globally, iodine deficiency remains an important cause in some regions, as iodine is needed to make thyroid hormone.
Other causes include treatment for an overactive thyroid, surgery to remove part of the thyroid, certain medications, and, less commonly, problems with the pituitary gland. Hypothyroidism can also occur during or after pregnancy.
Diagnosis and treatment
Hypothyroidism is diagnosed with a simple blood test that measures thyroid-stimulating hormone (TSH) and thyroid hormone levels. A high TSH with low thyroid hormone confirms an underactive thyroid. Because symptoms overlap with many other conditions, this blood test is the reliable way to diagnose it.
Treatment is usually a daily tablet of levothyroxine, a synthetic version of the thyroid hormone your body lacks. Once the right dose is established through follow-up blood tests, most people feel completely well and lead normal lives. Treatment is generally lifelong, and it is important to take the medication consistently and attend regular reviews.
Living with an underactive thyroid
Once treatment with levothyroxine is established at the right dose, most people with hypothyroidism feel well and lead entirely normal lives. The key to living well with the condition is consistency: taking your medication every day, usually on an empty stomach and at the same time, and attending regular blood-test reviews so your dose stays correct over time.
Levothyroxine is best absorbed away from food, so it is typically taken in the morning at least 30 to 60 minutes before breakfast, or at night well after eating. Certain supplements and medicines, including iron, calcium and some indigestion remedies, can interfere with its absorption, so these should be taken several hours apart. Mentioning all your medications and supplements to your doctor or pharmacist helps avoid interactions.
Your thyroid needs can change over time — with age, weight change, pregnancy or other illnesses — which is why periodic monitoring continues even when you feel well. If symptoms return or change, a blood test can check whether your dose needs adjusting. With this straightforward routine, an underactive thyroid is very compatible with a full, active life.
Hypothyroidism, pregnancy and weight
Thyroid function is particularly important during pregnancy, both for the mother and the developing baby, who relies on the mother's thyroid hormone in early pregnancy. Women with hypothyroidism often need their dose adjusted during pregnancy and require closer monitoring, so it is important to inform your doctor as soon as you are pregnant or planning to be. Untreated or poorly controlled hypothyroidism in pregnancy can carry risks, which is why careful management matters.
Weight is a common concern with an underactive thyroid. Because the condition slows metabolism, it can cause modest weight gain, often partly from fluid retention. However, the gain attributable to hypothyroidism is usually limited, and treatment typically reverses only part of it. Significant or lasting weight management still depends on the same balanced diet and regular activity that benefit everyone.
It is worth having realistic expectations: treating hypothyroidism restores normal thyroid hormone levels and resolves many symptoms, but it is not a weight-loss treatment. If weight remains a concern after your thyroid levels are well controlled, a doctor or dietitian can offer tailored advice.
Understanding the causes more fully
Knowing why the thyroid becomes underactive helps make sense of the condition and its management. In countries where dietary iodine is sufficient, the most common cause is an autoimmune condition called Hashimoto's thyroiditis, in which the immune system gradually attacks and damages the thyroid gland, reducing its ability to make hormone. This process is usually slow, which is why symptoms develop gradually over months or years.
Globally, iodine deficiency remains an important cause, because iodine is an essential building block of thyroid hormone. Other causes include previous treatment for an overactive thyroid, surgery to remove part or all of the thyroid, radiotherapy to the neck, and certain medications. Less commonly, a problem with the pituitary gland — which signals the thyroid — can lead to an underactive thyroid.
Hypothyroidism can also appear during or after pregnancy in some women, and newborns are routinely screened for it because early treatment is so important for development. Understanding the cause occasionally affects management, but for the great majority of people, regardless of the cause, the treatment is the same: daily levothyroxine to replace the missing hormone, adjusted to the right dose through blood tests.