What Is an Underactive Thyroid? Symptoms & Treatment

An underactive thyroid, known medically as hypothyroidism, is a condition in which the thyroid gland does not produce enough thyroid hormones to meet the body’s needs. Because these hormones influence nearly every organ system, the effects range from persistent fatigue and weight gain to slowed heart rate and impaired memory. The condition is treatable, usually with a single daily pill, but diagnosis and management are more nuanced than most people realize.

What the Thyroid Actually Does

Your thyroid is a small, butterfly-shaped gland at the front of your neck. It produces two hormones: thyroxine (T4) and a smaller amount of triiodothyronine (T3). T4 is the main circulating form, and your body converts it into T3, the more biologically active hormone, in tissues throughout the body. Together, these hormones regulate how fast your cells burn energy. They affect your heart rate, body temperature, digestion, muscle strength, and even how quickly you think.

The system is tightly regulated by a feedback loop between the brain and the thyroid. When hormone levels drop, the pituitary gland at the base of the brain releases more thyroid-stimulating hormone (TSH) to prod the thyroid into producing more T4 and T3. When levels rise, TSH falls. This feedback is highly nonlinear, meaning the pituitary reacts with surprisingly large TSH swings in response to small shifts in thyroid hormone levels, which is part of what makes TSH such a sensitive diagnostic marker.¹

Why Thyroids Become Underactive

The most common cause in developed countries is Hashimoto’s thyroiditis, an autoimmune disorder in which the immune system attacks the thyroid gland. Multiple defects in regulatory immune cells lead to a loss of self-tolerance, meaning the body stops recognizing its own thyroid tissue as “self.”² The resulting infiltration of immune cells gradually destroys the gland’s ability to make hormones. Hashimoto’s affects women roughly seven to ten times more often than men and is driven by a mix of genetic susceptibility, environmental triggers, and the composition of the gut microbiome.³ Blood tests often reveal elevated antibodies against thyroid peroxidase (TPO-Ab) or thyroglobulin (TG-Ab), which are characteristic markers of the disease.⁴

Other causes include surgical removal of part or all of the thyroid (often for cancer or large goiters), radioactive iodine treatment for an overactive thyroid, radiation therapy to the head or neck, and severe iodine deficiency. Certain medications also suppress thyroid function directly. Lithium, widely prescribed for bipolar disorder, has well-documented antithyroid effects. Immune-modulating drugs used for cancer, hepatitis, and inflammatory conditions can trigger hypothyroidism through both immune and non-immune pathways.⁵ If you start any of these medications, your doctor should monitor your thyroid levels periodically.

Recognizing the Symptoms

Hypothyroidism tends to develop slowly, sometimes over years, which is part of why people often attribute its symptoms to aging, stress, or depression. The classic signs include:

• Fatigue: A deep, persistent tiredness that does not improve with rest. Thyroid dysfunction is one of the first things doctors consider when evaluating unexplained fatigue.⁶
• Weight gain: Usually modest, on the order of five to fifteen pounds, caused mostly by fluid retention and a slower metabolism rather than large-scale fat accumulation.
• Cold intolerance: Feeling chilly when others around you are comfortable.
• Constipation: Slowed gut motility is a direct consequence of reduced thyroid hormone.
• Dry skin and hair: Skin may become rough and flaky, and hair can thin or become brittle.
• Muscle weakness and joint aches: Low thyroid hormone slows protein turnover in muscles.
• Mental sluggishness: Difficulty concentrating, forgetfulness, and a general sense of mental cloudiness sometimes described as “brain fog.”

These symptoms overlap with many other conditions, and no single symptom is unique to hypothyroidism. That overlap is exactly why blood tests are essential for diagnosis rather than relying on symptoms alone.

How It Affects the Heart

What many people do not realize is that hypothyroidism has pronounced effects on the cardiovascular system. It can alter cardiac contractility, raise vascular resistance, increase blood pressure, and disturb heart rhythm.⁷ Even mild, subclinical hypothyroidism has been linked to impaired heart function at rest and during exertion, as well as a heightened risk of atherosclerosis.⁸ For someone already at elevated cardiovascular risk, untreated hypothyroidism adds an extra burden that is entirely reversible once hormone levels are corrected.

Getting Diagnosed

Diagnosis starts with a blood test measuring TSH. Because the pituitary responds so sensitively to thyroid hormone levels, an elevated TSH is usually the earliest sign that the thyroid is struggling, often rising before hormone levels have dropped enough to cause obvious symptoms. The standard reference range for TSH is roughly 0.45 to 4.5 mIU/L, though the exact upper cutoff remains debated.⁹

A few important caveats make TSH less straightforward than it sounds. Your TSH can shift with time of day, time of year, age, ethnicity, medications, supplements, iodine intake, and even whether you smoke.¹⁰ Older adults tend to have naturally higher TSH levels, which means applying the same cutoff to a 25-year-old and an 80-year-old can lead to overdiagnosis in the older person. When TSH is elevated, doctors typically follow up with a free T4 measurement. If free T4 is low, that confirms overt hypothyroidism. If free T4 is still within the normal range despite a high TSH, the diagnosis is subclinical hypothyroidism.

There is also a measurement limitation that most patients never hear about. Standard laboratory assays occasionally miss true hypothyroidism. One study found that all patients appeared to have normal thyroid function by standard immunoassay, yet when hormone levels were measured using a more precise technique, every patient showed biochemical hypothyroidism.¹¹ This is an edge case rather than a common problem, but it helps explain why a small number of people with clear symptoms are told their labs are “normal.”

The Subclinical Gray Zone

Subclinical hypothyroidism occupies an awkward middle ground. Your TSH is elevated, but your free T4 is still within range, and you may or may not have symptoms. The question of whether to treat it with medication depends on how high the TSH is and what other health risks you carry. Current evidence supports starting levothyroxine when TSH rises above 10 mIU/L.¹² Below that threshold, the picture is murkier. Younger patients with cardiovascular risk factors may benefit from treatment, while in older adults, a mildly elevated TSH may simply reflect normal aging and treating it aggressively can do more harm than good.

One frustrating reality is that symptoms like fatigue, weight gain, and reduced quality of life associated with subclinical disease often persist even after levothyroxine brings TSH back into range.¹² That does not mean the medication failed; it may mean those symptoms had other causes all along. Doctors are encouraged to explore alternative explanations rather than simply increasing the dose.

Standard Treatment with Levothyroxine

The standard treatment is levothyroxine (synthetic T4), taken as a daily tablet on an empty stomach. Your body converts the T4 into T3 as needed, just as it would with natural thyroid hormone. This approach became the standard of care in the 1970s, when researchers discovered that the body’s own conversion enzymes (deiodinases) could reliably generate T3 from T4, and when the development of the TSH blood test revealed that many patients on older thyroid extracts were being overtreated.¹³

Timing matters. Levothyroxine is absorbed in the small intestine, and its bioavailability ranges from about 60% to 80% under fasting conditions. If you eat within an hour of taking it, absorption drops and is delayed.¹⁴ That is why the classic advice is to take your pill first thing in the morning with water, then wait at least 30 to 60 minutes before eating or drinking coffee. Calcium, iron supplements, and antacids are particularly notorious for interfering with absorption and should be separated by several hours.¹⁵ Some people find it easier to take levothyroxine at bedtime, well after their last meal, and studies suggest this can work just as well as morning dosing.

The dose is individualized, typically starting low and increasing based on follow-up TSH checks every six to eight weeks. Once stable, most people need just one or two blood tests per year to confirm they are on the right dose. The goal is to bring TSH into the normal range without pushing it too low, which would create an overactive state with its own health risks.

When Levothyroxine Alone Is Not Enough

Most people feel well on levothyroxine monotherapy. But a subset of patients report persistent symptoms, including fatigue, brain fog, and difficulty with weight, despite having normal TSH levels on treatment.¹⁶ This has led to ongoing interest in combination therapy, which adds a small dose of synthetic T3 (liothyronine) alongside T4.

The research on combination therapy is mixed. Randomized trials have generally not shown clear superiority over levothyroxine alone for most measured outcomes. A recent meta-analysis found that combination therapy lowered scores on a general health questionnaire, suggesting patients felt somewhat better, but found no significant difference in heart rate, cholesterol, TSH levels, or depression scores.¹⁷ Another meta-analysis found that combination therapy significantly improved LDL cholesterol and showed a borderline trend toward weight reduction, while maintaining normal TSH levels.¹⁸ The evidence is not strong enough for guidelines to recommend combination therapy as first-line, but many endocrinologists will consider a trial of added T3 in patients who remain symptomatic despite optimal TSH levels.

Desiccated thyroid (made from pig thyroid glands) is an older preparation that naturally contains both T4 and T3. Before synthetic hormones existed, it was the only option. Some patients and practitioners still prefer it, though its T4-to-T3 ratio differs from what the human thyroid produces, and the dosing is less precise than with synthetics.¹⁹

Brain Fog That Lingers

A growing area of research focuses on the cognitive complaints that persist in some treated patients. “Brain fog” is not a formal medical term, but the experience it describes is real: difficulty with word retrieval, poor short-term memory, slowed processing speed, and a feeling that thinking requires more effort than it should. Studies confirm that a meaningful number of levothyroxine-treated patients report these symptoms even when their blood work looks normal.²⁰²¹

The cause is not fully understood. One theory is that some tissues rely on local T3 conversion and that standard blood tests do not reflect hormone levels inside the brain. Another possibility is that the autoimmune inflammation in Hashimoto’s itself causes symptoms independent of hormone levels. For now, the honest answer is that the science has not caught up with the patient experience. If you are on levothyroxine with normal labs but still feel foggy, it is worth discussing with your doctor, but be wary of anyone claiming a guaranteed fix.

Hypothyroidism During Pregnancy

Pregnancy raises the stakes considerably. During the first trimester, the fetus depends entirely on the mother’s thyroid hormones for brain development. Insufficient maternal thyroid hormone during early pregnancy is a risk factor for impaired fetal neurodevelopment, and this risk extends through later pregnancy as well, potentially affecting IQ and school performance.²² A meta-analysis found that maternal subclinical hypothyroidism was associated with roughly double the odds of intellectual disability indicators in offspring.²³

What complicates matters is that the same meta-analysis found no clear evidence that treating maternal subclinical hypothyroidism with levothyroxine actually reduced the incidence of low IQ in children.²³ The association between low maternal thyroid hormone and developmental risk is real, but it has been surprisingly hard to prove that giving levothyroxine changes the outcome. Despite this uncertainty, most guidelines recommend monitoring thyroid levels during pregnancy and treating overt hypothyroidism aggressively. Women already on levothyroxine typically need their dose increased by about 30% to 50% during pregnancy because the body’s demand for thyroid hormone rises.

Children and Newborn Screening

Congenital hypothyroidism, present at birth, is the most common preventable cause of intellectual disability in children.²⁴ Left untreated, it leads to severe developmental delay. Fortunately, newborn screening programs now test for it within the first few days of life in most countries. When treatment starts promptly, children go on to have broadly normal neurocognitive outcomes.²⁵ This is one of the great success stories of preventive medicine.

Older Adults and Overtreatment

In older adults, the risk-benefit calculation tilts differently. TSH naturally drifts upward with age, and a level that would be flagged as abnormal in a 30-year-old may be perfectly normal in someone over 70. Treating these naturally higher levels with levothyroxine can push someone into a mildly overactive state, which in older adults carries real dangers. Excessive replacement in elderly patients is associated with increased mortality.²⁶ One longitudinal study found that thyroid hormone initiation was highest among women over 80, and for about a third of treated participants with follow-up data, overtreatment persisted for at least two years.²⁷ This suggests that many older adults on thyroid medication are getting more hormone than they need, and their doctors are not catching it quickly enough.

Nutrients the Thyroid Needs

Iodine is the raw ingredient for thyroid hormone. In much of the developed world, iodized salt has largely eliminated iodine deficiency, but it remains a problem in some regions and in people who avoid salt or eat highly restrictive diets. Beyond iodine, several other nutrients play supporting roles. Selenium protects the thyroid gland from oxidative damage during hormone production and supports the deiodinase enzymes that convert T4 to T3. Iron is needed for thyroid peroxidase, the enzyme that incorporates iodine into thyroid hormones; iron deficiency blunts the effectiveness of iodine supplementation.²⁸ Zinc also supports enzymatic activity, and vitamin D has emerged as a potential immunoregulatory factor in autoimmune thyroid conditions.²⁹

One important caution: in areas where both iodine and selenium deficiency are severe, correcting selenium before iodine can worsen hypothyroidism.²⁸ For most people in developed countries, a balanced diet provides adequate amounts of all these nutrients without supplements. Megadosing iodine in hopes of boosting thyroid function is a bad idea; excess iodine can actually trigger or worsen autoimmune thyroid disease.

Myxedema Coma

If hypothyroidism is left untreated and reaches an extreme, it can progress to myxedema coma, a life-threatening emergency. Despite the name, patients are not always comatose, but they do show impaired consciousness, hypothermia, dangerously low blood pressure, and widespread organ dysfunction.³⁰ It is rare and almost always triggered by something else, like an infection, surgery, or a new medication, in someone whose hypothyroidism was already severe and uncontrolled. Treatment requires intensive care with intravenous thyroid hormone and management of whatever triggered the decompensation.³¹ Mortality is high even with treatment, making it a compelling reason not to ignore a hypothyroidism diagnosis.

When Abnormal Thyroid Labs Are Not Really Hypothyroidism

People who are seriously ill from other causes, whether an infection, a heart attack, or major surgery, frequently show abnormal thyroid function tests without actually having thyroid disease. This is known as nonthyroidal illness syndrome (sometimes called euthyroid sick syndrome). The pattern typically shows low T3 with normal or low TSH, which can look confusingly like hypothyroidism on paper.³² The changes appear to be the body’s adaptive response to severe illness rather than a sign that the thyroid is failing. The key distinguishing features are the presence of severe non-thyroidal illness and elevated cortisol.³³ Treating these patients with thyroid hormone is generally not recommended and can be harmful. Once the underlying illness resolves, thyroid labs typically return to normal on their own.

This matters because hospitalized patients routinely get thyroid levels checked, and a doctor unfamiliar with the syndrome might start levothyroxine unnecessarily. If you or someone you know receives a hypothyroidism diagnosis during a hospital stay for a serious unrelated illness, it is worth asking whether the abnormal labs could simply reflect the body’s stress response rather than true thyroid failure.