The 2026 Levels Guide to Thyroid Health

This tiny gland affects nearly every cell in your body. Here’s how to keep it functioning at its best to support your overall health.

WRITTEN BY

Nicola Bridges

REVIEWED BY

Angela Mazza, MD

Updated: 02/12/2026|10 min read

Despite its small size, the thyroid greatly influences overall health. Located at the front of the neck, this butterfly-shaped endocrine gland produces hormones that affect almost every cell in your body—regulating metabolism, body temperature, energy levels, mood, and reproductive health. So when your thyroid isn’t functioning properly, the ripple effects can be far-reaching.

Thyroid disorders affect an estimated 20 million people in the U.S., yet 60 percent are unaware that something’s amiss. These disorders are frequently overlooked or misdiagnosed because many symptoms—such as fatigue, mood swings, and weight changes—could be attributed to numerous conditions, and because physicians don’t routinely check thyroid functioning. Yet research links hypothyroidism and hyperthyroidism to cardiovascular diseases, diabetes, autoimmune disorders, and more.

This guide will help you understand how your thyroid works, recognize signs of possible dysfunction, navigate testing options, and keep your thyroid functioning at its best.

What is the thyroid and how does it function?

The thyroid is a vital endocrine gland that produces the following hormones:

• Triiodothyronine (T3): Considered the ‘active’ thyroid hormone because it binds to thyroid hormone receptors, triggering cellular responses, T3 has the greatest effect on metabolism. It helps regulate the rate at which the body uses calories.
• Thyroxine (T4): Though it’s the main hormone produced by the thyroid, T4 is inactive and doesn’t affect metabolism. However, through a process called deiodination, certain organs can convert T4 to T3 so it can be utilized.
• Reverse triiodothyronine (rT3): Another inactive form created through deiodination when T4 loses a different iodine atom, rT3 appears to block active T3 from binding to receptors. About one-third of all T4 is converted to rT3. In a healthy person, the ratio of T3 to rT3 should be less than 10:1. Research suggests that rT3 increases during starvation and illness to preserve energy.
• Calcitonin: Lowers calcium levels in the blood.

Thyroid function operates through an intricate feedback system called the hypothalamic-pituitary-thyroid (HPT) axis. This system works 24/7 to maintain thyroid hormone levels within a precise range for optimal growth, development, and metabolism:

• The hypothalamus region of the brain releases thyrotropin-releasing hormone (TRH).
• TRH stimulates the pituitary gland to produce thyroid-stimulating hormone (TSH).
• TSH signals the thyroid gland to produce and release T3 and T4 into the bloodstream.
• This process requires iodine, an essential trace element sourced from food or supplements, which is a building block for T3 and T4.

What are signs of thyroid dysfunction?

Most thyroid conditions develop gradually over time, with subtle symptoms often overlooked until the situation becomes serious. The most common thyroid conditions are:

Hypothyroidism

Hypothyroidism results from an underactive thyroid gland that doesn't produce enough thyroid hormones. This can lead to symptoms such as weight gain, persistent fatigue, increased cold sensitivity, constipation, depression, dry skin, muscle pain, and thinning hair. It’s often slow to develop and easily mistaken for other conditions—including anxiety, depression, chronic fatigue syndrome, and fibromyalgia—or the natural aging process. Treatment for hypothyroidism typically involves lifelong synthetic T4 hormone replacement with an oral medication called levothyroxine to help restore metabolic balance. Liothyronine (synthetic T3) and dessicated thyroid extract (DTE) are also sometimes used.

Hyperthyroidism

Hyperthyroidism results from an overactive thyroid gland producing too many hormones. This can cause anxiety, irritability, rapid or irregular heartbeat, shortness of breath, unexplained weight loss despite increased appetite, heat intolerance, sweating, tremors in the hands, and difficulty sleeping. Treatments include anti-thyroid medications like methimazole or propylthiouracil (PTU), which reduce production of T3 and T4; radioactive iodine therapy to destroy overactive thyroid cells; and thyroidectomy, a surgery to remove all or part of the thyroid gland.

Thyroid nodules

These abnormal lumps or growths inside the thyroid gland are quite common but usually benign. Typically, doctors monitor them with blood tests and ultrasounds to track size changes. Potential signs of thyroid cancer include rapid growth, irregular borders, and small flecks of calcium (microcalcification). An ultrasound and biopsy can assess these.

Autoimmune thyroid diseases

These occur when the immune system mistakenly attacks your thyroid tissue.

• Hashimoto's thyroiditis: The most common cause of hypothyroidism, Hashimoto’s progresses slowly over time. It causes chronic inflammation and the production of antibodies that attack and gradually destroy the thyroid gland’s ability to produce hormones. Progressing slowly over years, symptoms range from constipation, fatigue, and muscle weakness to hair loss, puffiness around the eyes, and a goiter (swelling of the thyroid causing the neck to bulge). The primary treatment is hormone replacement with levothyroxine.
• Graves' disease: The most common cause of hyperthyroidism, Graves’ disease happens when the immune system produces antibodies that stimulate the thyroid gland to produce excessive amounts of thyroid hormones. Symptoms include thyroid eye disease (TED) and skin thickening on the shins (thyroid dermopathy). TED and pretibial dermopathy are Graves-specific conditions separate from other causes of hyperthyroidism. Primary treatments include antithyroid medications and radioiodine therapy to destroy thyroid gland cells.

How does the thyroid impact health?

Thyroid hormones influence virtually every system in the body and can significantly impact, as well as be impacted by, several health conditions.

Autoimmune disorders

There's a strong link between autoimmune thyroid diseases and other autoimmune conditions due to shared genetic predispositions and immune dysregulation. For example, people with diabetes, celiac disease, Type 1 diabetes, vitiligo, and alopecia have a higher incidence of developing thyroid dysfunction.

Cardiovascular health

Because they influence genes that manage cardiovascular tone, calcium concentration, and muscle contraction, thyroid hormones affect heart rate, heart contraction strength, and blood pressure. Hypothyroidism can elevate cholesterol levels, increase blood pressure, and decrease heart rate, while hyperthyroidism can cause rapid heart rate, arrhythmias (like atrial fibrillation), and thyrotoxic cardiomyopathy, a type of dysfunction that can lead to heart failure.

Insulin resistance

Thyroid hormones play a crucial role in glucose metabolism and, in turn, the risk of insulin resistance and Type 2 diabetes. In particular, T3 appears in animals at least to enhance the body’s response to insulin, increasing glucose uptake by the cells, which can help maintain insulin sensitivity. Both insufficient and excessive thyroid hormone production can lead to dysfunction, though.

Hypothyroidism can reduce glucose uptake by the cells, impair insulin signalling pathways, and promote visceral fat, all of which can contribute to the development of insulin resistance. Hyperthyroidism can also lead to insulin resistance: Increased production of thyroid hormones stimulates the liver to produce more glucose, which can lead to hyperglycemia (high blood sugar levels).

Mental health conditions

Hypothyroidism can manifest as depression, fatigue, apathy, and cognitive decline, while hyperthyroidism can cause difficulty concentrating, anxiety, irritability, nervousness, and panic attacks. It’s unclear why, but a few things may be at play. Thyroid hormones influence levels of neurotransmitters that regulate mood (such as serotonin) and alertness (such as norepinephrine). Additionally, inflammation associated with hypo- and hyperthyroidism may hinder brain function, leading to cognitive impairment.

Metabolic health

Thyroid dysfunction may contribute to metabolic syndrome (MetS), which, in turn, may alter thyroid function. Thyroid hormones are metabolism’s primary regulators. They control the speed of food conversion into energy, influencing basal metabolic rate (the rate at which the body burns calories at rest), directly impacting energy use, weight management, and fat distribution. As noted above, thyroid hormones also influence glucose metabolism, cholesterol levels, and blood pressure, three other contributing MetS risk factors.

Bone and muscle

Thyroid hormones play a role in bone density and bone turnover (the ongoing replacement and repair of bone tissue). Hyperthyroidism can lead to an increased risk of osteoporosis. In muscles, thyroid hormones are involved in energy production needed to contract and relax.

Reproductive health

In women, thyroid hormones influence the hypothalamic-pituitary-gonadal (HPG) axis, which regulates the release of estrogen, progesterone, LH (luteinizing hormone), and FSH (follicle-stimulating hormone). If these hormones are imbalanced due to thyroid disorders, it can cause menstrual irregularities, infertility, and increased risk of miscarriage, as well as polycystic ovarian syndrome (PCOS).

The HPG axis also regulates testosterone production. So, in men, thyroid disorders may impact fertility by affecting sperm count and quality and may impair sexual function, which may lead to erectile dysfunction.

Does thyroid health change with age?

Yes, aging impacts thyroid health.

• Childhood: Thyroid function is critical in early development and growth, essential for brain development and cognitive function. Untreated, childhood thyroid disorders can lead to developmental delays.
• Adolescence: Hormonal fluctuations, puberty, and growth spurts can sometimes unmask or increase underlying thyroid conditions.
• Adulthood: The thyroid typically maintains stable function through adulthood. However, doctors often misattribute normal decreases in metabolism (which start around age 60) to aging when they may be due to a thyroid imbalance.
• Menopause: Postmenopausal women face an increased risk of thyroid dysfunction due to the natural decline in estrogen levels. This hormone increases the production and decreases the clearance of thyroid-binding globulin (TBG), which binds to T3 and T4. Less estrogen means less TBG, more free T3 and T4, and possibly symptoms of hyperthyroidism. However, thyroid conditions can be challenging to diagnose, since symptoms overlap with perimenopause symptoms.

Thyroid tests your doctor may order

Most doctors don’t recommend routine thyroid screening. Testing is essential if you have signs of thyroid dysfunction, a family history of thyroid disease, or any autoimmune conditions. Doctors typically follow a cascade testing approach, first testing TSH and then following up with additional tests to dig deeper depending on the consecutive results.

TSH test

The thyroid-stimulating hormone (TSH) test is the primary screening test for thyroid dysfunction. High TSH levels (above 5.0 mU/mL) can indicate hypothyroidism, and low TSH levels (below 0.5 mU/mL) can indicate hyperthyroidism.

T3 and T4 hormone level tests

These tests measure total and free T3 and T4. Free hormones are floating freely in the bloodstream, unbound to proteins in the blood, but the total amount (free and bound) is typically a more accurate measure to help confirm a diagnosis of hypo- or hyperthyroidism.

Thyroid antibody tests

Considered crucial for diagnosing autoimmune thyroid conditions, these tests detect specific antibodies: anti-thyroid peroxidase antibodies (TPOAb) for Hashimoto's thyroiditis, thyroid-stimulating immunoglobulins (TSI), or Thyroid-stimulating hormone receptor antibodies (TRAb) for Graves' disease.

Imaging studies

A thyroid ultrasound can evaluate the thyroid gland's structure and size, detect and characterize nodules, and assess signs of thyroid cancer. A radioactive iodine scan looks at how well the nodules absorb iodine to detect increased or decreased activity. It’s particularly useful for determining the cause of hyperthyroidism (for example, Graves' disease versus toxic nodules).

How to protect and manage your thyroid health

Everyday lifestyle changes that benefit overall health can also help keep your thyroid functioning well.

• Eat a balanced diet rich in iodine and selenium: The thyroid needs sufficient iodine to produce T4 and T3. Good sources include seafood (like cod and tuna), dairy products, eggs, and iodized salt. However, excessive iodine (above 1.1 milligrams per day) can be harmful, so balance is key. Selenium is a trace mineral necessary for converting T4 to active T3 and protecting the thyroid from oxidative damage. Great sources include fish and shellfish (such as tuna and shrimp), lean meats (chicken, turkey, and pork), and Brazil nuts.
• Manage stress: Chronic increased cortisol (the stress hormone) inhibits TSH secretion, and hypothyroidism may lead to elevated cortisol levels, creating a feedback loop. Studies suggest that stress-reducing practices such as yoga, deep-breathing exercise, and guided imagery may help.
• Exercise regularly: Physical activity helps regulate weight, reduce stress, and improve energy levels, which all indirectly benefit thyroid health. Research suggests that aerobic exercise, resistance training, and a combination of the two improve physical quality of life and T4 levels in women with hypothyroidism, while doing both increases TSH and mental quality of life. Another study reported that any vigorous activity lasting about 20 minutes can elevate TSH.
• Sleep well: Sleep deprivation can disrupt hormone regulation, potentially impacting thyroid function by reducing TSH production and causing an imbalance of the HPT axis.
• Avoid environmental toxins: Exposure to pesticides, heavy metals, and endocrine-disrupting chemicals (found in some plastic personal care products and food packaging) can interfere with thyroid function. Organic foods, filtered water, and natural personal care products may help support thyroid health.
• Supplement zinc and vitamin D: Zinc is involved in thyroid hormone synthesis and regulation, so that deficiency can impair thyroid function. Good sources include oysters, nuts, and lean red meat. Zinc supplements can be helpful if your diet falls short. Vitamin D deficiency has been linked to an increased risk of autoimmune thyroid diseases, particularly Hashimoto's thyroiditis. There’s continuing debate about the benefits of supplemental vitamin D on thyroid health, so talk this over with your healthcare provider.

Additionally, emerging technologies may help improve thyroid health management. Researchers are working to identify genetic predispositions and markers associated with thyroid disease susceptibility, and to create personalized hormone replacement regimens based on genetic makeup, lifestyle, and disease characteristics. For example, combination therapies (T4 and T3) may help some hypothyroid patients who don’t feel optimal on standard levothyroxine therapy.

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