Hypothyroidism (Underactive Thyroid)

What the thyroid does

The thyroid is a butterfly-shaped gland in the front of the neck. It makes thyroxine (T4) and triiodothyronine (T3), hormones that set the pace of metabolism in nearly every cell: heart rate, body temperature, energy use, digestion, mood and growth. The pituitary gland controls the thyroid by releasing thyroid-stimulating hormone (TSH). When thyroid hormone is low, the pituitary makes more TSH, so a high TSH is the hallmark of primary hypothyroidism.

Hypothyroidism symptoms

Symptoms usually develop slowly over months to years and are often blamed on aging, stress or busy lives.

Common symptoms

• tiredness and sluggishness
• weight gain (usually modest, a few kilograms) and fluid retention
• feeling cold when others are comfortable
• constipation
• dry, rough skin and brittle nails
• thinning hair, including the outer eyebrows
• muscle aches, cramps and stiffness
• joint pain
• low mood, depression, poor memory and slowed thinking ("brain fog")
• heavy, irregular or prolonged periods
• hoarse voice
• puffy face and eyelids
• slow heart rate

Other signs

• carpal tunnel syndrome and tingling hands
• high cholesterol
• an enlarged thyroid (goiter)
• infertility or miscarriage

Hypothyroidism in women

Women are 5 to 8 times more likely to develop hypothyroidism. It can affect periods, fertility and pregnancy, and it often appears after childbirth (postpartum thyroiditis) or around menopause, when symptoms can overlap with menopausal changes.

In older adults

Symptoms may be subtle: confusion, depression, falls or heart problems rather than classic signs.

In children

Slowed growth, delayed puberty, poor school performance and tiredness. Babies are screened at birth for congenital hypothyroidism, which causes intellectual disability if untreated.

Causes

• Hashimoto's thyroiditis: the immune system gradually destroys the thyroid. The most common cause in iodine-sufficient countries. Runs in families and occurs with other autoimmune conditions such as type 1 diabetes, celiac disease, pernicious anemia and vitiligo.
• Treatment for an overactive thyroid: radioactive iodine or surgery to remove the thyroid.
• Thyroid surgery for nodules, goiter or cancer.
• Radiation to the neck for cancer.
• Medicines: lithium, amiodarone, some cancer immunotherapies (checkpoint inhibitors), interferon, and tyrosine kinase inhibitors.
• Thyroiditis: temporary hypothyroidism after viral or postpartum thyroiditis.
• Iodine deficiency: the leading cause worldwide, rare in the US.
• Pituitary or hypothalamic problems (central hypothyroidism), rare.
• Congenital hypothyroidism: an absent or underdeveloped thyroid at birth.

Diagnosis

Other tests:

• thyroid peroxidase (TPO) antibodies: confirm Hashimoto's
• cholesterol and blood count
• thyroid ultrasound if the gland is enlarged or lumpy

TSH can be temporarily abnormal during illness, so tests are often repeated. Biotin supplements can interfere with thyroid tests; stop them for a couple of days before testing.

Treatment

Levothyroxine

Levothyroxine is a synthetic form of T4, identical to the natural hormone. It is taken once a day for life in most cases.

• the dose depends on weight, age, heart health and the cause
• older adults and people with heart disease start low and increase slowly
• TSH is rechecked 6 to 8 weeks after starting or changing the dose, then every 6 to 12 months once stable
• symptoms improve over weeks; full benefit can take a few months

How to take it

• take it at the same time every day, on an empty stomach with water, usually 30 to 60 minutes before breakfast (or at bedtime, at least 3 hours after eating)
• separate it by 4 hours from calcium, iron, magnesium, antacids and some cholesterol drugs, which block absorption
• coffee and soy can reduce absorption
• stick with the same brand or formulation where possible, and recheck TSH after a switch

Too much or too little

Too much levothyroxine can cause a racing heart, anxiety, tremor, sleep problems and, over years, bone loss and atrial fibrillation. Too little leaves symptoms unresolved. Regular TSH testing keeps the dose right.

Other options

Liothyronine (T3) or combination T4/T3 therapy and desiccated thyroid extracts are used in selected people who remain symptomatic despite normal TSH, though evidence of benefit is limited.

Pregnancy

Thyroid hormone is essential for the baby's brain development. Women on levothyroxine usually need about a 25 to 30 percent higher dose as soon as pregnancy is confirmed. TSH is checked every 4 weeks in the first half of pregnancy.

Complications if untreated

• high cholesterol and heart disease
• infertility, miscarriage and pregnancy complications
• depression and cognitive problems
• goiter
• nerve damage
• myxedema coma: a rare, life-threatening state of extreme hypothyroidism with low temperature, drowsiness and organ failure, often triggered by infection or cold in older adults

Frequently asked questions

What are the first signs of hypothyroidism?

Early signs are often vague: tiredness, feeling cold, weight gain, constipation, dry skin and low mood. They develop slowly, so people often attribute them to stress or aging. A TSH blood test can quickly check.

What causes an underactive thyroid?

In the US, Hashimoto's thyroiditis, an autoimmune condition, is the most common cause. Others include thyroid surgery or radioactive iodine treatment, neck radiation, some medicines such as lithium and amiodarone, temporary thyroiditis, and iodine deficiency worldwide.

Can hypothyroidism be cured?

Most hypothyroidism, including Hashimoto's, is permanent and needs lifelong levothyroxine. Some forms, such as postpartum or viral thyroiditis and medicine-induced hypothyroidism, are temporary and may resolve. Treatment fully controls symptoms for most people.

Does hypothyroidism cause weight gain?

It can, but usually modestly, a few kilograms, much of it from fluid. Treatment typically reverses this partly. Significant weight gain usually has other contributors, so diet and activity remain important.

How long does it take for levothyroxine to work?

Levels start to rise within days, but most people feel better over 2 to 6 weeks, and it can take a few months for all symptoms to improve. TSH is rechecked after 6 to 8 weeks to adjust the dose.

What is the difference between hypothyroidism and hyperthyroidism?

Hypothyroidism is too little thyroid hormone, causing slowness, tiredness, weight gain and feeling cold. Hyperthyroidism is too much, causing a racing heart, anxiety, weight loss, tremor and heat intolerance. Both are diagnosed with thyroid blood tests.