Hypothyroidism Explained: Underactive Thyroid Symptoms, Causes and Diagnosis
The thyroid is a small, butterfly-shaped gland at the front of the neck. It makes two main hormones, thyroxine (T4) and triiodothyronine (T3), which influence how the body uses energy. They affect heart rate, body temperature, digestion, muscle strength, and many other functions. Hypothyroidism, or an underactive thyroid, means the gland is not making enough of these hormones.
According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), part of the NIH, hypothyroidism affects roughly 5 in 100 Americans age 12 and older, although many cases are mild. It is more common in women and in people over 60. This guide covers what causes it, the symptoms people notice, and how it is diagnosed. It is educational only.
How the thyroid is controlled
The thyroid works under the direction of the pituitary gland in the brain. The pituitary releases thyroid-stimulating hormone (TSH), which tells the thyroid to make T4 and T3. When thyroid hormone levels fall, the pituitary usually makes more TSH to push the thyroid harder. This feedback loop is why a raised TSH level is often the first sign of an underactive thyroid.
Common causes
- Hashimoto’s disease. The most common cause in the United States, according to NIDDK. It is an autoimmune condition in which the immune system attacks the thyroid, gradually reducing its ability to make hormones. It often runs in families.
- Thyroiditis. Inflammation of the thyroid, which can follow pregnancy (postpartum thyroiditis) or a viral infection. It may cause a temporary overactive phase followed by an underactive phase.
- Treatment for other thyroid conditions. Radioactive iodine treatment, thyroid surgery, or radiation to the neck can reduce or remove thyroid function.
- Certain medicines. Some medications, such as lithium and amiodarone, can affect thyroid function.
- Congenital hypothyroidism. Some babies are born with a thyroid that does not work properly, which is why newborns are screened in many countries.
- Iodine imbalance. The thyroid needs iodine to make hormones. Iodine deficiency is uncommon in the United States because of iodized salt, but too much iodine can also cause problems in some people. The NIH Office of Dietary Supplements notes that both too little and too much can affect thyroid function.
- Pituitary problems. Rarely, the problem lies with the pituitary gland rather than the thyroid itself.
Symptoms
Hypothyroidism usually develops slowly, so symptoms can be easy to overlook or attribute to aging or stress. Commonly listed symptoms include:
- Tiredness and low energy
- Feeling cold more easily
- Weight gain
- Constipation
- Dry skin and thinning or dry hair
- Muscle aches, weakness, or joint pain
- Slowed heart rate
- Heavier or irregular menstrual periods
- Low mood or depression
- Memory problems or slowed thinking
- A swollen thyroid (goiter) in some people
None of these symptoms is specific to hypothyroidism on its own. Many people with these symptoms have normal thyroid function, and some people with mild hypothyroidism have no symptoms at all. That is why blood tests are needed.
Who may be at higher risk
NIDDK and the NHS list several factors associated with higher risk: being female, being over 60, a family history of thyroid disease, having another autoimmune condition (such as type 1 diabetes or celiac disease), having been pregnant or given birth in the past six months, past thyroid surgery or radioactive iodine treatment, and radiation to the neck or upper chest.
How hypothyroidism is diagnosed
A clinician will typically ask about symptoms and medical history, examine the neck, and order blood tests:
- TSH test. Usually the first test. A high TSH suggests the thyroid is underactive.
- Free T4 test. Measures the amount of active thyroxine in the blood. A high TSH with a low free T4 indicates overt hypothyroidism.
- Thyroid antibody tests. Antibodies such as thyroid peroxidase (TPO) antibodies can suggest Hashimoto’s disease.
When TSH is mildly raised but free T4 is normal, this is called subclinical hypothyroidism. Whether and when it needs treatment depends on the TSH level, symptoms, age, pregnancy plans, and other factors. Clinicians often repeat tests after several weeks, because TSH can vary and can be affected by recent illness. For more on testing, see our guide to thyroid blood tests.
Why diagnosis matters
Untreated hypothyroidism can contribute to raised cholesterol and, in severe cases, to heart problems. During pregnancy, thyroid hormone is important for the baby’s development, so people who are pregnant or planning pregnancy are often checked more closely. Very severe, untreated hypothyroidism can rarely cause a life-threatening condition called myxedema coma, which needs emergency care.
How it is managed
Hypothyroidism is commonly managed with thyroid hormone replacement prescribed and monitored by a clinician, with doses adjusted based on follow-up blood tests. Management is individual. Things that can affect how well replacement works include timing relative to food, other medications and supplements (such as iron or calcium), pregnancy, and weight changes. People should not start, stop, or change thyroid medication or take thyroid “support” supplements without talking to their clinician. Some over-the-counter supplements marketed for thyroid health have been found to contain actual thyroid hormone or large amounts of iodine.
Hypothyroidism at different life stages
Pregnancy and after birth. Thyroid hormone needs rise during pregnancy, and some people develop thyroid problems for the first time after giving birth. Postpartum thyroiditis can cause a short overactive phase followed by an underactive phase, and in many cases function returns to normal within about a year, though some people develop lasting hypothyroidism.
Older adults. In people over 60, symptoms may be subtle or mistaken for normal aging, such as fatigue, memory changes, or feeling cold. TSH levels also tend to rise slightly with age, which clinicians take into account when interpreting results.
Children and teens. In young people, an underactive thyroid can affect growth, puberty, and school performance, so unexplained changes are worth discussing with a pediatrician.
Questions to ask your clinician
- What do my TSH and free T4 results mean, and what range is right for me?
- Do I have antibodies suggesting Hashimoto’s disease?
- Could any of my medications or supplements be affecting my results?
- How often should my levels be rechecked?
- If I am planning pregnancy, does anything need to change?
- Which of my symptoms might not be caused by my thyroid?
The bottom line
Hypothyroidism is common and usually develops gradually. Its symptoms overlap with many other conditions, so blood tests interpreted by a clinician are essential. If you have symptoms or risk factors, ask your clinician whether thyroid testing makes sense for you.
Sources
- NIDDK (NIH): Hypothyroidism (Underactive Thyroid)
- Mayo Clinic: Hypothyroidism – Symptoms and causes
- NHS: Underactive thyroid (hypothyroidism)
- Endocrine Society: Thyroid
- NIH Office of Dietary Supplements: Iodine Fact Sheet
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Thyroid Blood Tests Explained: TSH, Free T4, T3 and Antibodies
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