What hypothyroidism means
Hypothyroidism, sometimes called an underactive thyroid, happens when the thyroid does not make enough hormone. Thyroid hormones affect how the body uses energy and help the brain, heart, muscles, and other organs work normally.
Symptoms and common causes
Early symptoms can be vague and may be confused with aging, menopause, stress, or other conditions. Symptoms can include tiredness, feeling cold, dry skin, constipation, weight gain, slowed heart rate, low mood, and changes in hair or menstrual periods.
Hashimoto's disease is a common cause. Other causes include thyroid surgery, radioactive iodine or radiation treatment, thyroid inflammation, and some medicines.
Testing and diagnosis
A clinician usually starts with thyroid blood tests. TSH, or thyroid-stimulating hormone, is commonly checked first. A high TSH most often points to hypothyroidism, but the full picture may include free T4 and, when appropriate, thyroid antibody tests. Learn how the main thyroid blood tests fit together.
Symptoms alone cannot determine whether someone has hypothyroidism. Test results need to be interpreted alongside medical history, medicines, and the reason testing was ordered.
Treatment and follow-up
When treatment is needed, it commonly uses levothyroxine, a synthetic form of thyroid hormone. The dose and follow-up testing are individualized. Do not start, stop, or change thyroid medicine without discussing it with a clinician.
Tell the prescribing clinician about pregnancy or plans for pregnancy, other medicines, and supplements such as calcium, iron, or biotin because they can affect testing or how thyroid medicine is taken.
Questions to ask your clinician
- Which thyroid tests do I need, and what do they show?
- What is the likely cause of my thyroid hormone level?
- Do I need treatment now, and what is the goal of treatment?
- When should my blood tests be rechecked?
- Could any medicines or supplements affect my results or treatment?
- What should I discuss if I am pregnant or planning pregnancy?
Sources and medical review
Medically reviewed by Richard B. Guttler, MD, FACP, FACE, ECNU · September 2026
This guide draws on Dr. Guttler's earlier patient education and the current medical references below.