Learn About Underactive Thyroid Signs and Symptoms

What Is an Underactive Thyroid and How Does It Work

Your thyroid is a small, butterfly-shaped gland located in the front of your neck, just below your Adam's apple. This gland produces hormones that control how your body uses energy. When your thyroid works correctly, it releases the right amount of hormones to keep your metabolism running at a healthy pace. Your metabolism is the process your body uses to convert food into energy.

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An underactive thyroid, also called hypothyroidism, occurs when your thyroid gland does not produce enough thyroid hormone. The main hormone involved is called thyroxine, or T4. When T4 levels drop below normal, your body's metabolic rate slows down. This means your cells work more slowly, and your body burns fewer calories at rest. This condition affects roughly 4.6% of the U.S. population, though rates are higher among older adults and women.

Your pituitary gland, located at the base of your brain, helps control thyroid function. When T4 levels are low, the pituitary releases thyroid-stimulating hormone (TSH) to signal the thyroid to produce more T4. In hypothyroidism, the thyroid does not respond properly to this signal, or it cannot produce enough hormone no matter how much TSH the pituitary releases.

Several conditions can cause an underactive thyroid. Hashimoto's thyroiditis is an autoimmune disease where your immune system attacks thyroid tissue and is the most common cause in regions with adequate iodine intake. Iodine deficiency causes hypothyroidism in other parts of the world. Other causes include surgical removal of part or all of the thyroid, radiation therapy to the neck or head, certain medications, pregnancy-related thyroid problems, and pituitary or hypothalamus disorders.

Practical Takeaway: Understanding that hypothyroidism involves reduced hormone production helps you recognize why symptoms develop. The condition is manageable once identified, and treatment can restore normal hormone levels and metabolism.

Common Physical Symptoms to Recognize

The physical symptoms of an underactive thyroid develop gradually, often over months or years. Because the condition slows your metabolism, the most common symptom is fatigue. People with hypothyroidism report feeling tired even after sleeping eight hours or more. This fatigue is not the kind relieved by rest alone—it comes from your body's reduced energy production at the cellular level.

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Weight gain is another frequent symptom. Since your metabolism slows, you burn fewer calories throughout the day. People with untreated hypothyroidism may gain 10 to 20 pounds or more without significantly changing their eating habits or exercise level. The weight gain often occurs despite reduced appetite in some cases.

Cold sensitivity represents another classic sign. Your thyroid hormones regulate body temperature. With reduced hormone levels, your body cannot maintain its normal temperature as effectively. You may feel cold even in warm environments, prefer warmer clothing than others around you, or have difficulty warming up after exposure to cold. Some people develop a condition called cold intolerance, where they cannot tolerate cold at all.

Other physical symptoms include:

• Muscle weakness and aches
• Joint pain and stiffness, especially in hands and feet
• Heavy menstrual periods or irregular periods in women
• Constipation due to slowed digestive function
• Dry skin that may feel rough or scaly
• Hair loss or thinning, particularly from the outer edges of the eyebrows
• Slowed heart rate (bradycardia)
• Puffy face, hands, or feet from fluid retention
• A hoarse or deeper voice than usual

Practical Takeaway: Recognizing these physical symptoms in yourself or others provides important information to discuss with a healthcare provider. No single symptom proves hypothyroidism exists, but a pattern of several symptoms together warrants medical evaluation.

Mood and Cognitive Changes Associated with Hypothyroidism

An underactive thyroid significantly affects mental and emotional health. Depression is one of the most common psychological symptoms, occurring in up to 40% of people with hypothyroidism. This depression often does not respond well to antidepressant medications alone until the thyroid condition is treated. The depression associated with hypothyroidism tends to develop slowly and may feel like a general loss of interest in activities you once enjoyed, persistent sadness, or a sense of hopelessness.

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Anxiety can also develop or worsen with an underactive thyroid, though it is less common than depression. Some people experience both depression and anxiety together. The anxiety may manifest as worry, nervousness, or panic-like symptoms. Unlike anxiety from other causes, anxiety from hypothyroidism typically improves significantly once thyroid hormone levels are restored to normal.

Cognitive symptoms, sometimes called "brain fog," include difficulty concentrating, memory problems, and slower thinking. You may find it harder to focus on tasks, remember names or appointments, or process information quickly. Some people describe this as feeling mentally cloudy or like their thoughts move through molasses. These symptoms can affect work performance and daily functioning. Studies show that hypothyroidism can reduce processing speed and working memory capacity.

Mood changes may include irritability, even when situations do not normally bother you. Your patience may feel shorter, or you may become frustrated more easily. Some people experience mood swings. Sleep disturbances are also common—either excessive sleepiness during the day or insomnia at night, despite feeling tired. These sleep problems can compound fatigue and worsen other symptoms.

Practical Takeaway: Mental and cognitive symptoms of hypothyroidism are real biological changes, not personal failings. If you experience new depression, anxiety, brain fog, or personality changes alongside other hypothyroidism symptoms, discussing thyroid testing with your doctor can help determine the underlying cause.

Who Is at Higher Risk for Hypothyroidism

Certain groups of people have higher rates of hypothyroidism. Women are five to eight times more likely than men to develop this condition. The reasons include hormonal differences and higher rates of autoimmune thyroid disease in women. Pregnancy can trigger thyroid problems—about 5% of women develop thyroid dysfunction during or shortly after pregnancy, called postpartum thyroiditis.

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Age increases risk significantly. People over 60 have higher hypothyroidism rates than younger adults. A study of older adults found that over 10% of people in their seventies had hypothyroidism. The risk continues to increase with age, particularly in women. Screening for hypothyroidism is sometimes recommended for adults over 60, even without symptoms, since older adults may have subtle symptoms that are easy to overlook or attribute to normal aging.

Family history matters substantially. If your parents, siblings, or other close relatives have hypothyroidism or other autoimmune thyroid diseases, your risk is higher. Genetic factors make certain people more susceptible to developing autoimmune thyroid disease. Having other autoimmune conditions, such as type 1 diabetes, celiac disease, or rheumatoid arthritis, also increases hypothyroidism risk.

Medical treatments can increase risk. People who receive radiation therapy to the head, neck, or chest for cancer treatment have higher hypothyroidism rates, sometimes years after treatment. Thyroid damage from radiation can occur even decades later. Certain medications including lithium (used for bipolar disorder), some heart medications, and high-dose interferon can trigger hypothyroidism. People with iodine deficiency have higher rates, though this is rare in developed countries with iodized salt.

Other risk factors include a history of thyroid surgery or thyroid disease, recent pregnancy or miscarriage, and high stress levels. Some research suggests that chronic stress may contribute to developing autoimmune thyroid disease, though the exact mechanism remains under study.

Practical Takeaway: If you fall into higher-risk categories, becoming familiar with hypothyroidism symptoms helps with early recognition. Discussing your risk factors with a healthcare provider can inform decisions about screening.

How Hypothyroidism Is Diagnosed and Evaluated

A blood test is the only reliable way to diagnose hypothyroidism. The test measures TSH and T4 (and sometimes

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