The thyroid gland functions as a key regulator of your body's metabolism, which is essentially the rate at which your body uses energy. It does this by producing two primary hormones: thyroxine (T4) and triiodothyronine (T3). These hormones travel through the bloodstream and influence the activity of almost every cell in the body.

The production of thyroid hormones is controlled by the pituitary gland, a pea-sized gland at the base of the brain. The pituitary releases a hormone called thyroid-stimulating hormone (TSH) when it senses that thyroid hormone levels are too low. When levels are sufficient, TSH production decreases. This feedback system keeps thyroid hormones within a healthy range under normal circumstances.

Thyroid hormones affect heart rate, body temperature, breathing rate, how quickly food moves through the digestive tract, muscle strength, menstrual cycles, bone health, cholesterol levels, and mood. Because of how many functions they influence, a disruption in thyroid hormone production can create symptoms that seem diverse and disconnected, which is one reason thyroid disorders can be challenging to identify without testing.

The thyroid also plays a critical role in early childhood development, particularly in brain development and growth in infants and young children.

Hypothyroidism, also called an underactive thyroid, occurs when the thyroid gland does not produce enough thyroid hormone to meet the body's needs. With too little thyroid hormone, the body's processes slow down across the board, and a distinct set of symptoms tends to develop.

Hypothyroidism is the most common thyroid disorder. It is far more prevalent in women than in men, and its frequency increases with age.

The most common cause of hypothyroidism is Hashimoto's thyroiditis, an autoimmune condition in which the immune system mistakenly attacks the thyroid gland. Over time, this damage reduces the gland's ability to produce hormone. Other causes include prior thyroid surgery, radioactive iodine therapy, certain medications, and in some parts of the world, iodine deficiency.

Symptoms of hypothyroidism typically develop slowly and may include:

• Fatigue and sluggishness
• Unexplained weight gain
• Sensitivity to cold temperatures
• Constipation
• Dry skin and hair
• Hair loss or thinning
• Slowed heart rate
• Muscle weakness or aching
• Depression or low mood
• Brain fog or difficulty concentrating
• Puffiness in the face, particularly around the eyes
• Hoarse voice
• In women, heavier or irregular menstrual cycles

Because these symptoms are gradual and overlap with many other conditions, hypothyroidism is sometimes dismissed or attributed to aging, stress, or lifestyle factors. A simple blood test measuring TSH and thyroid hormone levels can confirm the diagnosis.

Hyperthyroidism, or an overactive thyroid, is the opposite condition. In hyperthyroidism, the thyroid gland produces too much thyroid hormone, causing the body's metabolism and many of its functions to accelerate.

The most common cause of hyperthyroidism is Graves' disease, an autoimmune condition in which the immune system stimulates the thyroid to overproduce hormone. Other causes include thyroid nodules that produce hormone independently and certain forms of thyroiditis that temporarily release stored hormones into the bloodstream.

Hyperthyroidism tends to produce a characteristic set of symptoms that reflect an overdriven metabolism:

• Unintentional weight loss despite a normal or increased appetite
• Rapid or irregular heartbeat (palpitations)
• Anxiety, nervousness, or irritability
• Tremors in the hands or fingers
• Excessive sweating and heat intolerance
• Frequent bowel movements or diarrhea
• Fatigue despite feeling wired
• Sleep difficulties
• Changes in menstrual cycles
• Enlargement of the thyroid gland (goiter)
• In Graves' disease, a distinctive protrusion of the eyes (exophthalmos)

Hyperthyroidism can have significant effects on the heart, bones, and overall well-being if left unaddressed. Like hypothyroidism, it is diagnosed through blood tests and a clinical evaluation.

Not all thyroid conditions involve hormone imbalances. Some involve physical changes to the gland itself. Two of the most common structural conditions are goiter and thyroid nodules.

A goiter is an enlargement of the thyroid gland. It can occur in the context of both hypothyroidism and hyperthyroidism, or when hormone levels are entirely normal. Goiters are often the result of the thyroid working harder than usual to maintain hormone production, which causes the gland to grow. Iodine deficiency, while rare in developed countries due to iodized salt, is a leading cause of goiter worldwide. Autoimmune thyroid diseases, certain medications, and other factors can also lead to goiter.

A small goiter may cause no symptoms at all. A larger goiter may create a visible or palpable swelling in the neck, a feeling of tightness, difficulty swallowing, or a hoarse voice.

Thyroid nodules are solid or fluid-filled lumps that form within the thyroid gland. They are extremely common and become more prevalent with age. Most thyroid nodules are benign and produce no symptoms. They are often discovered incidentally during imaging done for another reason.

In some cases, nodules can grow large enough to cause discomfort or to be visible in the neck. A small percentage of thyroid nodules may be cancerous, which is why providers typically recommend evaluation of newly discovered nodules to determine their nature. Evaluation may include blood tests, an ultrasound, and sometimes a biopsy.

Thyroid disorders can develop in anyone, but certain factors increase the likelihood.

• Being female significantly raises the risk. Women are diagnosed with thyroid disorders far more often than men, particularly autoimmune thyroid conditions like Hashimoto's thyroiditis and Graves' disease. The connection between thyroid function and female sex hormones may contribute to this pattern.
• Family history is an important factor. If a parent or sibling has a thyroid condition, particularly an autoimmune one, your own risk is elevated.
• Age plays a role as well. Thyroid disorders are more commonly diagnosed in adults over 60, though they can occur at any age. Thyroid conditions during pregnancy and in the postpartum period are also relatively common.
• Having another autoimmune condition, such as type 1 diabetes, rheumatoid arthritis, lupus, or celiac disease, increases the risk of developing an autoimmune thyroid disorder.
• Radiation exposure, particularly to the head and neck, and a history of thyroid surgery can also contribute to thyroid dysfunction.

Diagnosis typically begins with a review of symptoms, a physical examination of the neck to check for enlargement or lumps, and blood tests.

The TSH test is the primary screening test for thyroid dysfunction. TSH is the hormone produced by the pituitary gland to stimulate the thyroid. When the thyroid is underactive, the pituitary releases more TSH in an effort to prompt it to produce more hormone, so TSH levels are high in hypothyroidism. When the thyroid is overactive and producing too much hormone, TSH levels drop very low. An abnormal TSH result typically leads to follow-up testing that measures T3 and T4 levels directly.

Additional blood tests may check for thyroid antibodies, which can confirm an autoimmune thyroid condition.

Imaging, particularly a thyroid ultrasound, may be ordered to evaluate the size and structure of the gland, identify nodules, and assess their characteristics.

If nodules are found, a fine needle aspiration biopsy may be recommended to determine whether they are benign or require further attention.

The combination of a thorough history, physical exam, and appropriate lab work gives providers the information needed to make an accurate diagnosis and recommend an appropriate course of care.

Yes, and quite commonly. Because thyroid hormones affect so many systems in the body, thyroid disorders can produce symptoms that closely resemble those of other health conditions. This is one reason they are sometimes missed or delayed in diagnosis.

Hypothyroidism, with its fatigue, depression, weight gain, and cognitive fogginess, can be mistaken for depression, chronic fatigue syndrome, anemia, or simply the effects of aging or a busy lifestyle.

Hyperthyroidism, with its anxiety, rapid heartbeat, and weight loss, can be confused with anxiety disorders, heart arrhythmias, or other stress-related conditions.

The overlap is particularly notable in older adults, who may attribute many thyroid symptoms to aging, and in women going through perimenopause or menopause, whose symptoms can mirror those of thyroid dysfunction.

This is why thyroid blood tests are such a valuable tool. When symptoms are vague, persistent, or not responding to other explanations, testing thyroid function can quickly clarify whether the thyroid is contributing to the problem. In many cases, identifying and addressing a thyroid imbalance provides significant relief from symptoms that had resisted explanation.

You should seek evaluation if you experience a cluster of symptoms consistent with either hypothyroidism or hyperthyroidism and they have persisted for several weeks without a clear explanation. This includes prolonged unexplained fatigue, changes in weight, heart palpitations, changes in bowel habits, significant shifts in mood or cognition, or cold or heat intolerance that feels unusual for you.

Seek care promptly if you notice a visible lump or swelling in your neck, feel pressure or tightness in your throat, develop difficulty swallowing or a persistent hoarse voice, or notice changes in your eyes (particularly if they appear to be protruding).

If you have a family history of thyroid disease, another autoimmune condition, or have previously received radiation to your head or neck, discuss thyroid screening with your provider even if you have no current symptoms.

Thyroid disorders are very manageable when identified and addressed. A simple blood test is often all it takes to determine whether your thyroid is contributing to the way you feel.