Hypothyroidism — also called an underactive thyroid — is a condition in which the thyroid gland does not produce enough thyroid hormone to meet the body's needs. It is one of the most common endocrine disorders, affecting approximately 5% of Americans, with women and people over age 60 at highest risk.
What Causes Hypothyroidism?
The most common cause of hypothyroidism in the United States is Hashimoto's thyroiditis, an autoimmune condition in which the immune system mistakenly attacks thyroid tissue, gradually impairing its ability to produce hormone. Other causes include:
- Previous radioactive iodine treatment for hyperthyroidism
- Thyroid surgery that removed part or all of the gland
- Radiation therapy to the neck
- Certain medications (amiodarone, lithium, some immunotherapy drugs)
- Iodine deficiency (rare in developed countries)
- Pituitary or hypothalamic disorders (secondary hypothyroidism)
- Congenital hypothyroidism (present from birth)
Symptoms of Hypothyroidism
Because thyroid hormone affects nearly every organ system, the symptoms of hypothyroidism are wide-ranging. They often develop gradually, which can make diagnosis challenging. Common symptoms include:
- Persistent fatigue and sluggishness
- Unexplained weight gain or difficulty losing weight
- Feeling cold even in warm environments
- Constipation
- Dry skin and brittle nails
- Hair loss or thinning hair
- Slowed heart rate (bradycardia)
- Depression and mood changes
- Brain fog, difficulty concentrating, and memory problems
- Muscle weakness and joint pain
- Elevated cholesterol levels
- Heavier or irregular menstrual periods
- Swelling in the face and extremities (myxedema)
⚠️ Important: Many symptoms of hypothyroidism — fatigue, weight gain, depression — are non-specific and overlap with many other conditions. A blood test measuring TSH and thyroid hormone levels is the only reliable way to diagnose hypothyroidism.
How Is Hypothyroidism Diagnosed?
Diagnosis is based primarily on blood tests. The key tests include:
| Test | What It Measures | In Hypothyroidism |
|---|---|---|
| TSH (Thyroid-Stimulating Hormone) | Pituitary signal to the thyroid | Elevated |
| Free T4 | Active thyroid hormone level | Low or low-normal |
| Free T3 | Active form of thyroid hormone | May be low |
| TPO Antibodies | Marker for Hashimoto's | Elevated if Hashimoto's |
Subclinical Hypothyroidism
Some patients have a mildly elevated TSH but normal free T4. This is called subclinical hypothyroidism. Whether to treat subclinical hypothyroidism depends on the degree of TSH elevation, symptoms, age, pregnancy status, and presence of thyroid antibodies. An endocrinologist is best positioned to make this individualized decision.
Treatment
Hypothyroidism is typically treated with levothyroxine (L-T4), a synthetic form of T4 that is highly effective, safe, and well-tolerated. Treatment is usually lifelong. The dose is individualized and adjusted based on TSH levels, typically checked every 6–12 weeks after starting or changing treatment, and then annually once stable.
Key considerations in treatment include:
- Levothyroxine should be taken on an empty stomach, 30–60 minutes before food or other medications
- Certain supplements (calcium, iron, fiber) and medications can interfere with absorption
- Dose requirements change during pregnancy — notify your doctor immediately if you become pregnant
- Some patients may benefit from the addition of T3 (liothyronine) — discuss this with your endocrinologist
Key Takeaways
- Hypothyroidism is very common and is most often caused by Hashimoto's thyroiditis
- Symptoms are wide-ranging and develop gradually, often making diagnosis delayed
- Diagnosis requires blood tests — TSH is the most important screening test
- Levothyroxine is a highly effective, well-tolerated treatment that normalizes hormone levels
- Regular monitoring ensures your dose remains optimal over time