Conditions guide

Hypothyroidism

Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone. Because thyroid hormone helps control how the body uses energy, low levels can slow down many body functions.

Last updated 3 sources

At a Glance

  • The thyroid is a butterfly-shaped gland in the front of the neck.
  • In the US, the most common cause of hypothyroidism is Hashimoto's disease, an autoimmune condition.
  • Symptoms often develop slowly and can include tiredness, feeling cold, weight gain, and constipation.
  • Blood tests, especially TSH and free T4, are used to diagnose it.
  • It is commonly treated with thyroid hormone replacement medicine prescribed by a clinician.

What It Is

The thyroid gland makes hormones, mainly thyroxine (T4) and triiodothyronine (T3), that affect metabolism, heart rate, body temperature, and many other functions. The pituitary gland in the brain releases thyroid-stimulating hormone (TSH) to tell the thyroid how much hormone to make.

In hypothyroidism, the thyroid makes too little hormone. The pituitary often responds by releasing more TSH, which is why a high TSH level is a common sign of the condition. Some people have subclinical hypothyroidism, in which TSH is mildly high but thyroid hormone levels are still normal.

Hypothyroidism is more common in women and in people over 60. It is usually a long-term condition, but it can generally be managed well with treatment and regular monitoring.

Common Terminology

TSH (thyroid-stimulating hormone)
A hormone from the pituitary gland that signals the thyroid to make hormone; it is usually high in primary hypothyroidism.
Free T4
The portion of thyroxine in the blood that is not bound to proteins and is available for the body to use.
Hashimoto's disease
An autoimmune condition in which the immune system attacks the thyroid, gradually reducing hormone production.
Subclinical hypothyroidism
A mild form in which TSH is above the normal range but free T4 is still within the normal range.
Goiter
An enlarged thyroid gland, which can appear as swelling in the front of the neck.
Thyroid antibodies
Proteins, such as anti-TPO antibodies, that may be found in the blood when the immune system targets the thyroid.

Common Causes and Associations

Hypothyroidism has several possible causes. Some are related to the thyroid itself, and a few involve the pituitary gland.

  • Hashimoto's disease, the most common cause in the US.
  • Thyroiditis, or inflammation of the thyroid, including after pregnancy (postpartum thyroiditis).
  • Surgical removal of part or all of the thyroid.
  • Radioactive iodine treatment or radiation therapy to the neck.
  • Certain medicines, such as lithium and amiodarone.
  • Too little iodine in the diet, which is uncommon in the US but more common in some parts of the world.
  • Thyroid problems present at birth (congenital hypothyroidism) or, rarely, pituitary gland disorders.

Symptoms and Features

This is a general description of commonly reported features, not a diagnostic checklist. Symptoms tend to develop slowly, vary widely, and overlap with many other conditions.

  • Tiredness and low energy.
  • Feeling cold more easily than others.
  • Weight gain that is not otherwise explained.
  • Constipation.
  • Dry skin and thinning or dry hair.
  • Slowed heart rate, muscle aches, or joint stiffness.
  • Low mood, memory problems, or changes in menstrual periods.

Tests and Evaluation

Hypothyroidism is diagnosed mainly with blood tests, along with a review of symptoms and a physical exam.

  • TSH blood test, which is usually the first test used to check thyroid function.
  • Free T4 blood test, which helps confirm the diagnosis and its severity.
  • Thyroid antibody tests, which can help show whether Hashimoto's disease is the cause.
  • A physical exam, including feeling the neck for an enlarged thyroid or nodules.
  • Thyroid ultrasound in some cases, especially if the gland feels enlarged or lumpy.
  • Newborn screening, which checks babies in the US for congenital hypothyroidism.

Treatment Overview

Treatment aims to replace the missing thyroid hormone. Decisions about medication and monitoring are made with a healthcare professional.

  • The standard treatment is a synthetic form of thyroid hormone, most commonly levothyroxine, taken by mouth.
  • The amount of medicine is individualized by a clinician and is often adjusted based on follow-up TSH tests.
  • After the right amount is found, thyroid levels are typically rechecked periodically.
  • Some foods, supplements (such as iron and calcium), and other medicines can affect how thyroid medicine is absorbed, which is commonly discussed with a clinician or pharmacist.
  • Not everyone with subclinical hypothyroidism is treated; clinicians weigh test results, symptoms, and other factors.
  • Treatment is usually lifelong for most causes of hypothyroidism.

When Professional Care May Be Discussed

Because symptoms are often gradual and nonspecific, thyroid testing is frequently discussed during routine care.

  • Ongoing tiredness, feeling cold, weight changes, or other symptoms listed above are commonly discussed with a healthcare professional.
  • Swelling in the front of the neck or a lump in the thyroid area is usually evaluated by a clinician.
  • Pregnancy, or planning a pregnancy, is a time when thyroid function is often reviewed, since thyroid hormone is important for fetal development.
  • Changes in symptoms while on treatment are commonly discussed, since they may relate to thyroid levels.
  • Severe untreated hypothyroidism can rarely lead to myxedema coma, with symptoms like extreme drowsiness, confusion, and very low body temperature; this is treated as a medical emergency, and in the US, people are advised to call 911.

Questions to Ask a Healthcare Professional

General questions people often find useful to bring to an appointment:

  • What do TSH and free T4 results generally show about thyroid function?
  • What causes Hashimoto's disease, and is it inherited?
  • How is thyroid hormone replacement generally monitored over time?
  • What foods, supplements, or medicines can interfere with thyroid medicine?
  • What is subclinical hypothyroidism, and when is it treated?
  • How does hypothyroidism affect pregnancy?
  • How long does it usually take for symptoms to improve after treatment begins?

Sources

This guide was written in plain language from the following authoritative references.

  1. Hypothyroidism — MedlinePlus (U.S. National Library of Medicine)
  2. Hypothyroidism (Underactive Thyroid) — National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
  3. Underactive thyroid (hypothyroidism) — NHS

Last updated October 8, 2026. FreeDoc guides are general educational information and are not a substitute for care from a qualified professional.