Hypothyroidism, also called underactive thyroid or low thyroid, is a disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormone. Thyroid hormone regulates many important bodily functions, so a lack of it can affect multiple organs and body systems.
The primary organ affected by hypothyroidism is the thyroid gland itself. This small, butterfly-shaped gland located at the base of the neck produces thyroid hormones T3 and T4. In hypothyroidism, the thyroid is underactive and does not make enough hormones to meet the body’s needs.
Common causes of hypothyroidism include:
- Hashimoto’s thyroiditis: an autoimmune disease and the most common cause of hypothyroidism in the US. The body’s immune system attacks and damages the thyroid.
- Thyroid surgery: removal of part or all of the thyroid can lead to hypothyroidism.
- Radiation therapy: treatment for cancers of the head and neck can affect the thyroid.
- Congenital hypothyroidism: an absent or underdeveloped thyroid gland from birth.
- Medications: drugs like amiodarone, lithium, interferon alpha can damage the thyroid.
- Iodine deficiency: lack of dietary iodine impairs thyroid hormone production.
Regardless of the cause, hypothyroidism leaves the thyroid unable to manufacture sufficient hormones for the body’s metabolic needs.
Metabolism and Growth
Thyroid hormones regulate the body’s metabolism – all the biochemical processes that convert food and oxygen into energy. With inadequate thyroid hormones, metabolism slows down. This can lead to:
- Fatigue and weakness
- Weight gain or difficulty losing weight
- Intolerance to cold temperatures
- Muscle cramps
- Dry skin and hair
In children, hypothyroidism can affect growth and development. Lack of thyroid hormones can result in delayed puberty and impaired growth.
Thyroid hormones increase the heart’s pumping action and heart rate. With too little thyroid hormone, the heart rate slows down. This can lead to:
- Bradycardia – abnormally slow heart rate, less than 60 beats per minute
- Heart failure or enlarged heart (cardiomyopathy)
- Fluid around the heart (pericardial effusion)
- Narrowing of the heart valves
In severe cases, very low thyroid levels can even be life-threatening due to the effects on the heart.
Thyroid hormones are important for brain development and function. Untreated hypothyroidism can lead to:
- Memory and cognitive problems
- Fatigue and sluggishness
- Seizures (in babies with congenital hypothyroidism)
In infants and children, lack of thyroid hormones can cause cretinism – a rare condition characterized by permanent cognitive delays and growth impairments if not treated early.
The thyroid influences the reproductive system in both women and men. Hypothyroidism can cause:
- Irregular menstrual cycles and reduced fertility in women
- Impaired ovarian function
- Increased risk of miscarriage and preterm delivery
- Erectile dysfunction and reduced libido in men
Pregnant women with hypothyroidism are also at increased risk for preeclampsia, placental abruption, and postpartum hemorrhage.
Gastrointestinal problems are common in people with hypothyroidism. These include:
- Acid reflux
- Excessive gas
- Bloating and abdominal discomfort
- Diarrhea (less common)
The thyroid helps control the body’s use and absorption of nutrients. With low thyroid levels, digestion can slow down. Poor gut motility and low stomach acid can also contribute to GI problems.
Blood and Circulation
Thyroid hormones increase the production of red blood cells and prevent anemia. In hypothyroidism, reduced red blood cells can lead to:
- Anemia – fewer red blood cells than normal
- Impaired blood clotting
- Increased risk of bleeding
Low thyroid levels also cause high cholesterol and atherosclerosis which can impair blood circulation. This increases the risk of heart disease and stroke.
Although the connection is still unclear, hypothyroidism has been linked with reduced lung function and increased risk of respiratory disease. Possible respiratory effects include:
- Impaired breathing with exertion
- Worsening of pre-existing lung disease like asthma
- Increased risk of lung cancer
More research is needed to understand how thyroid hormones influence lung physiology and disease.
Thyroid hormones are critical for the growth and maturation of nerves. Hypothyroidism can lead to damage to nerves and impaired nerve signaling called neuropathy. Symptoms may include:
- Numbness and tingling in hands and feet
- Carpal tunnel syndrome
- Impaired reflexes and muscle control
- Hearing problems
If untreated in infants, hypothyroidism can cause severe developmental delays and mental retardation due to impaired brain and nerve development.
Bones and Joints
Thyroid hormones help build bone strength and prevent osteoporosis. With low thyroid levels, bones can become thin and brittle and joints may be affected. This can lead to:
- Osteopenia and osteoporosis
- Joint pain and stiffness
- Inflammation around joints
- Carpal tunnel syndrome
Early treatment can help prevent permanent bone and joint problems due to hypothyroidism.
Skin, Hair and Nails
Common skin, hair and nail symptoms of hypothyroidism include:
- Dry, coarse skin
- Pale, puffy face
- Hair loss or brittle, thinning hair
- Ridging or splitting of fingernails and toenails
With thyroid hormone controlling growth and repair, low levels can impair skin cell renewal, circulation, and sweating. This leads to dryness and loss of elasticity. Hair follicles and nail beds also have a high turnover of cells that require thyroid hormones.
Thyroid eye disease is a condition in which the muscles and fatty tissues behind the eye become inflamed. Common symptoms include:
- Dry, gritty, irritated eyes
- Puffy or retracted eyes
- Double vision
- Difficulty moving the eyes
- Watery eyes
- Light sensitivity
Although it often occurs with hyperthyroidism, thyroid eye disease can sometimes affect people with hypothyroidism as well. Proper thyroid treatment may help improve the condition.
The thyroid hormones T3 and T4 play a role in kidney function. With hypothyroidism, kidneys are unable to properly concentrate urine. This can lead to:
- Increased urination (polyuria)
- Constant thirst (polydipsia)
- Chronic kidney disease
- Urinary tract infections
Kidney problems due to hypothyroidism are usually reversible with proper thyroid hormone replacement therapy.
Thyroid hormones regulate the body’s immune response. Lack of these hormones can impair immunity. People with hypothyroidism are prone to:
- Frequent respiratory infections
- Yeast infections
- Urinary tract infections
- Slow wound healing
- Anemia unresponsive to iron
With thyroid replacement therapy, the immune system typically returns to normal.
In summary, hypothyroidism affects essentially every organ system due to the widespread actions of thyroid hormones in the body. While the thyroid gland itself is most directly impacted, many other body systems like the heart, brain, digestion, reproduction, muscles, nerves and immune function are also affected by low thyroid hormone levels.
Fortunately, with early diagnosis and proper treatment with thyroid hormone replacement medication, most of the effects of hypothyroidism on the body can be reversed. This allows the body’s organs to function normally again.