Underactive Thyroid: Causes, Symptoms & Treatment

An underactive thyroid is a common health problem, yet many people live with it for years before getting a clear diagnosis because the symptoms develop slowly and quietly. Feeling low, gaining weight, constant tiredness, or brain fog often gets blamed on normal life stress, so people adjust and move on, thinking this is just how life is now. In many cases, though, these signs point to one underlying issue that the thyroid gland is not producing enough hormones, causing the body to slow down bit by bit.

Identifying an underactive thyroid early is crucial because, if left untreated, it can lead to serious problems over time, affecting the heart, mental health, fertility, and overall quality of life. In this blog, we explore what hypothyroidism is, how common it is, its types, symptoms, causes, complications, diagnosis, treatment, daily care, and how to live well with the condition.

What is Hypothyroidism?

Hypothyroidism is a condition where the thyroid gland does not produce enough thyroid hormones. The thyroid gland is a small butterfly-shaped gland located in the front of the neck. Even though it is small, it controls how fast or slow many processes in the body work. Thyroid hormones help regulate metabolism, heart rate, body temperature, digestion, and brain function.

When hormone levels drop, the body starts to slow down. Organs do not get the signals they need to function properly. This is why people with hypothyroidism feel tired, cold, sluggish, or mentally slow.

From a medical point of view, hypothyroidism happens when the thyroid gland cannot respond properly to signals from the brain or when the gland itself is damaged. From a daily life point of view, it feels like the body is running on low battery most of the time.

How Common Is Hypothyroidism?

Hypothyroidism is common, and it is one of the more frequent thyroid disorders seen in clinical practice. It can affect people of any age, but it is more often diagnosed in women, and the risk tends to rise with age.​

Many cases are missed early because symptoms can look like other issues, such as stress, depression, sleep problems, or normal ageing. That is one reason an underactive thyroid can stay undiagnosed for a long time, even when symptoms are present.

Types of Hypothyroidism

Hypothyroidism is not the same for everyone. Doctors classify it based on where the problem starts in the body.

Primary Hypothyroidism

Primary hypothyroidism means the problem starts in the thyroid gland itself. This is the most common type. A common cause is Hashimoto’s disease, where the immune system slowly damages the thyroid over time. Because the gland cannot make enough hormones, the brain keeps sending stronger signals to push it to work.

As a result, TSH levels go up while thyroid hormone levels go down. Since thyroid hormones affect almost every part of the body, the symptoms can vary and may affect energy, weight, skin, mood, and digestion.

Secondary Hypothyroidism

Secondary hypothyroidism occurs when the pituitary gland in the brain does not release enough TSH. The thyroid gland may still be healthy, but it does not get the signal it needs to produce hormones.

In this case, both TSH and thyroid hormone levels may be low or not in the expected range. Doctors usually check for issues affecting the pituitary, such as growths, inflammation, or damage from injury or treatment. 

Tertiary Hypothyroidism

Tertiary hypothyroidism starts even higher up in the brain, in a part called the hypothalamus. This area normally tells the pituitary to release TSH, which then signals the thyroid. When the hypothalamus does not send the right message, the whole chain slows down. 

As a result, thyroid hormone levels drop. This type is rare and is usually looked at along with other brain-related hormone problems.

Subclinical Hypothyroidism

Subclinical hypothyroidism means blood tests show a raised TSH level, but thyroid hormone levels are still within the normal range. Many people feel fine, while others notice mild symptoms that are easy to ignore. 

Treatment depends on several factors, including how high the TSH level is, whether symptoms are present, pregnancy plans, heart health, and antibody levels. In some cases, doctors simply monitor the condition over time before deciding on treatment.

The Symptoms of Hypothyroidism

Symptoms of hypothyroidism often appear slowly. They may vary from person to person. Some people feel only mild discomfort, while others struggle with daily tasks.

Physical Symptoms

The body slows down when thyroid hormones are low. People often feel tired even after rest. Weight gain may happen without changes in diet. Skin becomes dry, hair turns brittle, and nails break easily. Constipation is common due to slow digestion.

Mental and Emotional Symptoms

Low hormone levels affect the brain, too. People may feel low, anxious, or depressed. Concentration becomes difficult. Memory feels weaker. Tasks that once felt easy may now feel exhausting.

Heart and Metabolism Changes

The heart rate may slow down. Cholesterol levels can rise, increasing the risk of heart disease. Blood pressure may also change over time.

Reproductive and Hormonal Symptoms

Women may experience irregular periods, heavy bleeding, or fertility problems. Men may notice reduced energy or low libido. During pregnancy, untreated hypothyroidism increases risks for both mother and baby.

What Are the Causes of Hypothyroidism?

Hypothyroidism develops for several reasons. Some cause damage to the thyroid directly, while others affect how it receives signals from the brain.

Hashimoto’s thyroiditis

This is the most common cause of hypothyroidism. It is an autoimmune condition where the immune system slowly attacks the thyroid gland. Over time, this damage reduces the gland’s ability to produce enough hormones.

Radioactive iodine treatment

Radioactive iodine is often used to treat an overactive thyroid. While it helps control excess hormone production, it can permanently reduce thyroid activity. Many people develop hypothyroidism after this treatment and require lifelong medication.

Thyroid surgery

Surgery to remove part or all of the thyroid may be done for nodules, cancer, or severe thyroid disease. When enough thyroid tissue is removed, hormone production drops, leading to hypothyroidism.

Radiation therapy

Radiation used for cancers of the head, neck, or chest can damage the thyroid gland. The effects may appear months or even years after treatment.

Medications

Some medicines interfere with thyroid hormone production or release. Long-term use increases the risk of developing hypothyroidism, which is why doctors monitor thyroid levels in such cases.

Iodine deficiency

Iodine is essential for the production of thyroid hormones. A lack of iodine reduces hormone production. Although iodised salt has lowered this risk, deficiency still exists in some regions.

Pituitary disorders

The pituitary gland sends signals that tell the thyroid how much hormone to make. Damage to the pituitary due to injury, infection, or growths can reduce these signals and lead to hypothyroidism.

Congenital disease

Some people are born with a missing or poorly functioning thyroid gland. This condition is present from birth and is usually detected through newborn screening. Early treatment is important for normal development.

Pregnancy

Hormonal changes during and after pregnancy can affect thyroid function. Some women develop hypothyroidism during this time, which may be temporary or long lasting.

De Quervain thyroiditis

This condition involves temporary inflammation of the thyroid, often after a viral illness. It may start with high thyroid hormone levels and later shift to hypothyroidism. Most people recover, though some may have lasting thyroid changes.

What Are the Complications of Hypothyroidism?

Complications usually develop when hypothyroidism is untreated or poorly controlled for a long time. The risk rises when symptoms are ignored or when medication is not taken consistently.

High cholesterol and heart disease risk

Low thyroid hormone can raise LDL cholesterol. Over time, this can increase cardiovascular risk. Some people also develop a slower heart rate.

Goitre

When the thyroid is struggling, it can enlarge due to constant stimulation. This swelling is called a goitre. It can cause a visible neck swelling or a tight feeling in the throat.

Mental health impact

Depression and cognitive slowing can become more pronounced if hypothyroidism stays untreated. Quality of life can drop due to persistent fatigue and poor concentration. Treatment often improves these symptoms when thyroid levels are corrected.

Infertility and pregnancy complications

Untreated hypothyroidism can affect ovulation and fertility. In pregnancy, uncontrolled hypothyroidism is linked with complications, so monitoring and dose adjustments are important. Many people have healthy pregnancies with proper treatment

Myxedema coma

Severe untreated hypothyroidism can rarely lead to myxedema coma, a life-threatening emergency. It can involve extremely low body temperature, slowed breathing, confusion, and loss of consciousness. It needs urgent hospital care.

How is Hypothyroidism diagnosed?

Diagnosis is based on symptoms, a physical exam, and blood tests. Because symptoms can be vague, testing plays a central role in confirming what is going on.

Medical history and physical exam

A doctor will ask about symptoms, family history, medicines, pregnancy status, and prior thyroid treatment. The exam may include checking heart rate, reflexes, skin texture, and the neck for thyroid enlargement. These clues guide the next tests.

TSH blood test

TSH is often the first test used to screen for hypothyroidism. In primary hypothyroidism, TSH is typically elevated because the pituitary is trying to push the thyroid to make more hormone. TSH interpretation can vary in central hypothyroidism, so doctors look at T4 too.

Free T4 blood test

Free T4 measures the amount of available thyroid hormone in the blood. Low free T4 with high TSH supports a diagnosis of primary hypothyroidism. Normal free T4 with high TSH suggests subclinical hypothyroidism.

Thyroid antibodies

Antibody tests can help identify autoimmune thyroid disease, such as Hashimoto’s. A positive test supports the diagnosis when the clinical picture fits. It can also help predict progression in some subclinical cases.

Additional tests when needed

If central hypothyroidism is suspected, doctors may evaluate other pituitary hormones and consider imaging. Ultrasound may be used when there is a goitre or nodules. The exact workup depends on symptoms and findings.

Treatment for Hypothyroidism

Treatment aims to restore thyroid hormone levels and reduce symptoms. Most people are treated with a daily tablet that replaces the missing hormone and works gradually over time.

Levothyroxine is the standard medicine used. The dose is tailored to each person based on blood tests, age, weight, pregnancy status, heart health, and symptoms. It often takes a few adjustments to find the right dose.

Blood tests are repeated after starting or changing treatment to ensure levels stay within range. Once stable, monitoring continues at regular intervals, especially during pregnancy, older age, or if heart disease is present.

Diet for Hypothyroidism

Diet plays a supporting role in managing hypothyroidism. While food cannot replace medical treatment, eating well helps maintain energy levels, support overall health, and manage some symptoms.

• Eat regular meals with a balance of whole grains, vegetables, fruits, and protein
• Include iodine-rich foods in safe amounts, such as dairy and iodised salt
• Add selenium-rich foods like nuts, seeds, and whole grains
• Choose zinc food sources such as legumes and seeds
• Limit highly processed foods and excess sugar
• Stay hydrated to support digestion and metabolism

Living With Hypothyroidism

Living with hypothyroidism is about long-term balance rather than quick fixes. Symptoms may not disappear overnight, and some days feel better than others. With regular treatment, awareness, and steady habits, most people can live full and active lives. The key is consistency and knowing how to respond to your body instead of ignoring it.

Building a Routine

A fixed daily routine helps keep thyroid hormone levels stable. Taking medicine at the same time each day improves how well it works. Regular blood tests and doctor visits help catch small changes early before symptoms return. Skipping doses or changing timing often leads to fatigue, low mood, or weight changes, even if it seems minor at first.

Emotional Support

Hypothyroidism can affect mood and mental clarity. Feeling low or emotionally flat is not uncommon. Understanding that these changes are part of the condition reduces self-blame. Support from family, friends, doctors, or counselors helps during difficult phases and makes the condition feel less isolating.

Wrapping Up

An underactive thyroid can quietly affect every part of life. The symptoms are often mistaken for normal stress or aging. Early diagnosis and consistent treatment make a real difference, helping prevent long-term complications and improving daily quality of life.

With the right medical care, lifestyle support, and regular monitoring, hypothyroidism becomes manageable. However, treatment is usually long-term and involves repeated doctor visits, blood tests, and ongoing medication. For many NRIs, managing a condition like hypothyroidism abroad can be costly. Treatment and follow-ups are often much more affordable in India. In such cases, having NRI health insurance by Niva Bupa can be a practical choice, offering coverage for consultations, diagnostics, and hospital care, while reducing financial strain during long term treatment.

FAQs

1. How often do I need thyroid tests once treatment starts?

   Most people get TSH checked about every 6 to 8 weeks after starting or changing levothyroxine until levels stabilize. After that, monitoring is often done every 6 to 12 months, unless symptoms change or there is a special situation like pregnancy.​

2. When should I take levothyroxine for the best results?

   Levothyroxine is usually taken once daily, 30 to 60 minutes before eating. Some medicines and supplements can reduce absorption, so doctors often advise spacing levothyroxine at least 4 hours apart from those products.​

3. Can calcium or iron supplements interfere with my thyroid medicine?

   Yes, calcium and iron can interfere with levothyroxine absorption if taken too close to the dose. Keeping a time gap helps the medicine work as intended.​

4. Can hypothyroidism raise cholesterol?

   Yes, hypothyroidism can raise LDL cholesterol levels. Treating hypothyroidism can improve cholesterol in many people, though some may still need separate cholesterol management.​