Hypothyroidism: Underactive Thyroid

Hypothyroidism means the thyroid gland is not making enough thyroid hormone for the body's needs. Because thyroid hormone helps set the pace of metabolism, an underactive thyroid tends to slow many body systems down. It is one of the most common hormone conditions and is usually straightforward to identify with a blood test.

What hypothyroidism is

The thyroid is a small, butterfly-shaped gland at the front of the neck. It produces two main hormones, thyroxine (T4) and triiodothyronine (T3), which travel through the bloodstream and influence nearly every tissue. These hormones help regulate how fast cells use energy, body temperature, heart rate, digestion, mood, and many other processes. When the supply falls short, those processes tend to slow, which is why the symptoms can touch so many parts of life.

The gland sits at the end of a control chain called the hypothalamic-pituitary-thyroid axis. The pituitary gland in the brain monitors how much thyroid hormone is circulating and releases thyroid-stimulating hormone (TSH) to keep the supply steady. When the thyroid cannot keep up, the pituitary responds by releasing more TSH to push it harder. This is why a raised TSH, combined with a low thyroid hormone level, is the classic signature of an underactive thyroid.

What causes it

Hypothyroidism can arise from a problem in the thyroid itself (primary hypothyroidism) or, far less commonly, from a problem with the pituitary or hypothalamus that fails to signal the thyroid properly (central hypothyroidism).

Hashimoto's thyroiditis

The most common cause in many parts of the world is an autoimmune condition called Hashimoto's thyroiditis. In this condition, the immune system gradually attacks thyroid tissue, reducing the gland's ability to make hormone over months or years. Because the change is slow, symptoms can creep up so gradually that they are easy to dismiss. Hashimoto's is often associated with thyroid antibodies that can be detected on a blood test, and it tends to run in families and to occur alongside other autoimmune conditions.

Other causes

• Treatment for a previously overactive thyroid, such as surgery or radioactive iodine
• Thyroid surgery for other reasons, including removal of part or all of the gland
• Certain medicines that affect thyroid function
• Radiation treatment to the head or neck
• Inflammation of the thyroid after pregnancy or a viral illness, which is sometimes temporary
• Iodine imbalance, since the thyroid needs iodine to make its hormones, with either too little or too much causing problems
• A condition present from birth (congenital hypothyroidism), which is why many newborns are screened
• Less commonly, a problem with the pituitary gland, which fails to release enough TSH

Common signs and symptoms

Because so many tissues depend on thyroid hormone, the symptoms are wide-ranging and develop gradually. People may notice some of the following:

• Persistent tiredness and low energy
• Feeling cold when others are comfortable
• Unexplained weight gain or difficulty losing weight
• Dry skin, brittle nails, and thinning hair
• Constipation
• Low mood, slowed thinking, or trouble concentrating
• Muscle aches, stiffness, or weakness
• A slower heart rate
• Heavier or irregular menstrual periods
• A hoarse voice or a visible swelling at the base of the neck (goitre)

No single symptom confirms hypothyroidism, and many people have only a few. These symptoms are common and non-specific, which is part of why a blood test matters so much for sorting out the cause.

How it is diagnosed

Diagnosis usually begins with a conversation about symptoms and a physical examination of the neck, followed by blood tests. The diagnosis rests on the blood work rather than symptoms alone, because the symptoms are so non-specific. The key tests are:

• TSH — the most sensitive first-line test. A high TSH suggests the thyroid is underactive. Our guide to the TSH test explains how the result is read.
• Free T4 — measures the available thyroid hormone. A low level alongside a high TSH supports the diagnosis of overt hypothyroidism.
• Thyroid antibodies — such as anti-thyroid peroxidase (anti-TPO) antibodies, which can point to an autoimmune cause such as Hashimoto's.

Sometimes the picture is mixed. A raised TSH with a normal free T4 is called subclinical hypothyroidism, discussed below. A low free T4 with a low or only-normal TSH may suggest a less common central cause and prompts a closer look at the pituitary.

The table below shows illustrative reference ranges only. Real ranges vary by laboratory, age, sex, and whether someone is pregnant, so interpret results with a clinician rather than against a generic chart.

You can read more about individual markers in our blood tests and hormones sections, or browse related symptoms.

Subclinical hypothyroidism

Subclinical hypothyroidism describes a mildly raised TSH while the free T4 level remains within range. In effect, the pituitary is working a little harder to keep thyroid hormone normal, but the thyroid is still managing to supply enough. Some people have no symptoms, while others have vague ones that may or may not be related.

Whether subclinical hypothyroidism is monitored or treated is an individual decision. Clinicians weigh how high the TSH is, whether symptoms are present, whether thyroid antibodies are detected, the person's age, and other health factors. In many cases the approach is simply to recheck the blood tests after a period of time, because mild elevations sometimes resolve on their own.

How it is generally managed

Hypothyroidism is generally managed by replacing the missing thyroid hormone so that levels return to a healthy range. The mainstay is a synthetic form of thyroxine taken daily, with the aim of relieving symptoms and normalising TSH over time. Because the right approach depends on the individual, clinicians typically recheck blood tests periodically, especially after any change, and adjust care accordingly.

Monitoring usually continues for life when the cause is permanent, since the goal is to keep thyroid hormone steady rather than to fix the gland. Temporary forms of thyroiditis may not need long-term treatment, and subclinical cases are sometimes monitored rather than treated immediately. Several factors, including other medicines, supplements such as biotin, and changes in body weight or pregnancy, can affect both the blood tests and how much hormone the body needs. General approaches to hormone conditions are described in our treatments section, but decisions about whether and how to treat are individual and belong with your clinician.

Pregnancy considerations

Thyroid hormone is especially important during pregnancy because it supports the developing baby, particularly in early pregnancy before the baby's own thyroid is working. The body's thyroid demands change during pregnancy, and the expected TSH range shifts as well, differing by trimester. For these reasons, thyroid status is often watched more closely before and during pregnancy in people with a known or suspected thyroid condition.

An untreated underactive thyroid during pregnancy can be associated with complications for both parent and baby, which is why clinicians monitor it carefully and may adjust care more frequently than at other times. Anyone who is pregnant or planning a pregnancy and has a thyroid condition should discuss monitoring with their clinician; this page describes the topic generally and is not a substitute for individual care.