Hypothyroidism: Causes, Symptoms, and Treatment
Hypothyroidism, an underactive thyroid gland, is one of the most common endocrine disorders, affecting an estimated 5% of the US population (and up to 10% when subclinical cases are included).
Medical ConditionsInformational
Written By: DocAi Health Editorial Team
Last Updated: 2026-07-15
Medically Reviewed By: DocAi Health Medical Review Team
Last Updated: 2026-07-15
Medically Reviewed By: DocAi Health Medical Review Team
Medical Disclaimer: This article is for general informational and educational purposes only and is not medical advice. It does not create a doctor-patient relationship and is not a substitute for professional diagnosis or treatment by a qualified healthcare provider. Never disregard or delay seeking professional medical advice because of something you have read here. If you think you may have a medical emergency, call 911 or your local emergency number right away. Health information can change and this guidance is general and US-focused, so consult a licensed clinician in your own country about your specific situation.
Hypothyroidism, an underactive thyroid gland, is one of the most common endocrine disorders, affecting an estimated 5% of the US population (and up to 10% when subclinical cases are included). The thyroid gland produces hormones that regulate virtually every metabolic process in the body, energy production, temperature regulation, heart rate, digestion, mood, and cognitive function. When thyroid hormone levels fall below what the body needs, virtually all these systems slow down. The symptoms are often nonspecific and overlap with many other conditions, which is why hypothyroidism is frequently misdiagnosed or goes undetected for years.
What Does the Thyroid Do?
The thyroid gland, located at the base of the neck, produces two main hormones: thyroxine (T4, the storage form) and triiodothyronine (T3, the active form). These hormones bind to receptors in nearly every cell in the body, regulating basal metabolic rate, protein synthesis, thermogenesis, heart function, and brain development. The pituitary gland regulates thyroid output by secreting thyroid-stimulating hormone (TSH): when thyroid hormone levels drop, TSH rises (signaling the thyroid to produce more); when levels are adequate or high, TSH drops. This is why TSH is the primary screening test for thyroid dysfunction.
Causes of Hypothyroidism
Hashimoto's Thyroiditis, Most Common Cause in the Developed World
Hashimoto's thyroiditis (also called Hashimoto's disease or chronic lymphocytic thyroiditis) is an autoimmune condition in which the immune system produces antibodies that attack and gradually destroy thyroid tissue. It accounts for the majority of hypothyroidism in iodine-sufficient countries. It is significantly more common in women (7:1 female-to-male ratio) and runs in families. Anti-thyroid peroxidase (anti-TPO) antibodies and anti-thyroglobulin antibodies are the markers tested to identify Hashimoto's. Hashimoto's can cause fluctuating thyroid function, sometimes with a temporary hyperthyroid phase ("Hashitoxicosis") before the hypothyroid stage develops.
Prior Thyroid Treatment
Thyroid surgery (partial or total thyroidectomy) and radioactive iodine (RAI) treatment, both used for hyperthyroidism or thyroid cancer, frequently cause hypothyroidism as a planned or incidental outcome.
Medications
Several medications can cause hypothyroidism: lithium (used for bipolar disorder), amiodarone (a heart rhythm medication with very high iodine content), and immune checkpoint inhibitors (used in cancer immunotherapy) are among the most common causes of drug-induced hypothyroidism.
Iodine Deficiency
The leading cause of hypothyroidism worldwide, iodine is essential for thyroid hormone synthesis. In iodine-sufficient countries like the US (where iodized salt is used), iodine deficiency hypothyroidism is rare except in very restricted diets or in pregnant women with inadequate iodine intake.
Central Hypothyroidism
Rarely, hypothyroidism results from pituitary or hypothalamic disease (failure to produce TSH or TRH that stimulates the thyroid), rather than from the thyroid itself. In central hypothyroidism, TSH may be normal or low despite inadequate thyroid function, a reason why free T4 is also measured in suspected pituitary disease.
Symptoms of Hypothyroidism
The symptoms of hypothyroidism reflect the slowing of metabolic processes throughout the body. They are often gradual in onset:
- Fatigue and persistent low energy (the most common presenting symptom)
- Weight gain or difficulty losing weight despite normal eating
- Cold intolerance, feeling cold when others are comfortable
- Constipation
- Dry skin, brittle nails
- Hair thinning or hair loss (diffuse; eyebrows, especially the outer third)
- Slowed heart rate (bradycardia)
- Depression, low mood, cognitive slowing ("brain fog")
- Heavy or irregular menstrual periods
- Muscle aches, weakness, and joint stiffness
- Hoarseness of voice
- Puffy face or peripheral edema
- Elevated cholesterol (thyroid hormones regulate LDL receptor activity)
When to Seek Medical Care
🚨 Emergency, Call 911
Seek emergency care immediately (call 911 or go to the nearest emergency room) if you or someone with known or suspected hypothyroidism develops:
- Confusion, extreme drowsiness, or unresponsiveness
- A very slow heart rate or very low blood pressure
- Very low body temperature (feels unusually cold to the touch, shivering has stopped)
- Difficulty breathing, or very slow/shallow breathing
- Fainting or loss of consciousness
These can indicate myxedema coma, a rare but life-threatening complication of severe, poorly managed hypothyroidism. It is most common in older adults and is often triggered by infection, cold exposure, or sedative medications. Do not wait to see whether symptoms pass on their own, call 911.
See a Doctor Soon (Not an Emergency)
If you notice new or worsening fatigue, weight gain, cold intolerance, hair loss, mood changes, or other symptoms above that developed gradually and are not severe or sudden, schedule an appointment with your healthcare provider rather than going to the ER.
Hypothyroidism can contribute to low mood or depression. If you are having thoughts of self-harm or suicide, call or text 988 (US), the Suicide & Crisis Lifeline, or call 911 if you are in immediate danger.
Diagnosis: Understanding TSH Results
The primary test for hypothyroidism is TSH (thyroid-stimulating hormone). In primary hypothyroidism, TSH is elevated (the pituitary is working harder to stimulate an underperforming thyroid). Free T4 is often measured to assess the degree of thyroid hormone deficiency. The normal TSH range is generally 0.4-4.0 mIU/L, though reference ranges vary slightly by laboratory.
Subclinical hypothyroidism: Elevated TSH with normal free T4, symptoms may be absent or mild. Treatment is recommended when TSH is above 10 mIU/L; for TSH 4-10, the decision depends on symptoms, antibody status, and individual factors. See the full TSH article for interpretation details.
Treatment: Levothyroxine
The standard treatment is levothyroxine (synthetic T4). At the correct dose, it fully replaces the thyroid hormone the body is not producing and resolves symptoms. Important points:
- TSH is rechecked 6-8 weeks after starting or adjusting the dose; full stabilization takes 3-6 months
- Most people feel significantly better within 4-8 weeks of reaching the appropriate dose
- Levothyroxine must be taken on an empty stomach, 30-60 minutes before food (see levothyroxine article)
- The goal TSH is usually 0.5-2.5 mIU/L for most patients, though this may be adjusted for age, pregnancy, or thyroid cancer
- Treatment is typically lifelong for autoimmune hypothyroidism
A small proportion of patients do not feel fully well on levothyroxine alone despite normal TSH, some respond to the addition of liothyronine (T3). This approach is not universally supported but can be considered in consultation with an endocrinologist.
Hypothyroidism and Special Populations
Pregnancy: Thyroid hormones are critical for fetal brain development. Untreated hypothyroidism in pregnancy is associated with miscarriage, preterm birth, and impaired fetal neurological development. Levothyroxine requirements increase by 25-50% in pregnancy, TSH should be checked at the first prenatal visit and each trimester.
Older adults: TSH targets may be set slightly higher (up to 4-6 mIU/L) in elderly patients, as overly aggressive treatment increases the risk of atrial fibrillation and bone loss.
Frequently Asked Questions
Can I have hypothyroidism with a normal TSH?
Primary hypothyroidism (the most common type) will show an elevated TSH. However, central hypothyroidism (pituitary or hypothalamic cause) can present with a normal or even low TSH despite low thyroid hormones. If pituitary disease is suspected, free T4 should also be measured. Symptoms alone without confirmed laboratory abnormality are generally not sufficient to diagnose hypothyroidism.
Does hypothyroidism go away on its own?
Hashimoto's thyroiditis is permanent, the immune destruction of thyroid tissue does not reverse. However, drug-induced hypothyroidism may improve when the offending medication is stopped. Postpartum thyroiditis (thyroid inflammation after delivery) may cause transient hypothyroidism that resolves spontaneously in some women, though risk of permanent hypothyroidism is higher if anti-TPO antibodies are present.
Is hypothyroidism the reason I can't lose weight?
Hypothyroidism contributes to modest weight gain through reduced metabolic rate and fluid retention. However, correcting hypothyroidism with levothyroxine typically results in only modest weight loss (2-4 kg), it does not "restart" weight loss for most patients with obesity. Weight management requires comprehensive lifestyle changes beyond thyroid correction.
Can diet or supplements fix hypothyroidism?
No evidence supports using dietary approaches or supplements (including iodine supplements) to treat established autoimmune hypothyroidism. Iodine supplementation can actually worsen autoimmune thyroid disease. The appropriate treatment is levothyroxine under medical supervision.
What is the connection between hypothyroidism and depression?
Thyroid hormone deficiency commonly causes depressive symptoms, fatigue, low mood, slowed thinking, and loss of interest. These symptoms can be identical to major depression. For this reason, thyroid function testing (TSH) is recommended as part of the workup for new or worsening depression. Treating hypothyroidism often resolves the depressive symptoms without antidepressants, though some patients require both. If you are having thoughts of self-harm or suicide, you do not need to wait for a thyroid diagnosis to get help: call or text 988 (US) (Suicide & Crisis Lifeline), or call 911 for immediate danger.
Related Articles:
Understanding Your TSH Result | Levothyroxine: Uses and Correct Administration | Causes of Fatigue | Causes of Unexpected Weight Changes
Understanding Your TSH Result | Levothyroxine: Uses and Correct Administration | Causes of Fatigue | Causes of Unexpected Weight Changes
Sources
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK, NIH): Hypothyroidism overview.
- MedlinePlus (National Library of Medicine, NIH): Hypothyroidism.
- American Thyroid Association: Hypothyroidism, guidelines and patient information.
- Mayo Clinic: Hypothyroidism, symptoms, causes, and treatment.
- Endocrine Society: Clinical practice guidelines for hypothyroidism.