T4, or thyroxine, is the main hormone produced by your thyroid gland. This small butterfly-shaped gland in your neck might not get much attention until something goes wrong, but it produces hormones that affect virtually every cell in your body.
In this article, you’ll learn what T4 is, what happens when it’s too high or too low, and what you can do about it.
What Exactly Is T4?
T4 is the thyroid hormone that’s most often tested to see if your thyroid gland is healthy. It’s produced in much larger quantities than its more active cousin, T3 (triiodothyronine). About 80% of the hormone output from your thyroid is T4, while only 20% is T3.
What makes T4 unique is that it primarily works as a "prohormone"—it gets converted into the more active T3 in various tissues throughout your body.
When T4 is too high or too low, it usually indicates a problem with your thyroid gland—and possibly even thyroid disease.
Why T4 Is So Important for Your Health
T4 and its active form T3 affect nearly every process in your body. Here's why maintaining healthy T4 levels is crucial.
Energy and Metabolism
T4 is essential for setting your metabolic rate—how quickly your body burns calories for energy. This affects everything from your weight to your energy levels.
People with low T4 often feel tired and cold and may gain weight easily, while those with excess T4 may experience weight loss despite increased appetite and feel anxious or overheated.
Brain Function
Your brain is particularly sensitive to thyroid hormone levels. T4 and T3 support memory, concentration, and mood stability. When T4 levels drop too low, many people experience brain fog, depression, and slowed thinking.
This is why "brain symptoms" are often the first clue something might be wrong with your thyroid.
Heart Health
T4 influences how fast and hard your heart beats. It affects your heart rate, blood pressure, and cholesterol levels. Both too much and too little T4 can cause heart problems, from irregular heartbeats to changes in blood pressure.
Growth and Development
During pregnancy and childhood, proper T4 levels are absolutely critical for normal development, especially brain development. Even mild thyroid deficiency in a pregnant woman can affect her child's IQ and neurological development.
Digestive Function
T4 helps regulate digestive processes and bowel movements. Low levels commonly cause constipation, while high levels can lead to more frequent bowel movements or diarrhea.
Reproductive Health
For women, normal T4 levels are necessary for regular menstrual cycles and fertility. For men, thyroid function affects testosterone production and sperm health. Both men and women may experience reduced sex drive when T4 levels are abnormal.
When T4 Levels Are Too High or Too Low
Problems with T4 production fall into two main categories: hypothyroidism and hyperthyroidism.
Hypothyroidism
When your thyroid doesn't produce enough T4, you develop an underactive thyroid or hypothyroidism.
This is the most common thyroid disorder, affecting millions of Americans—especially women over 60. Symptoms include fatigue, weight gain, cold intolerance, dry skin, constipation, and depression.
Hyperthyroidism
When your thyroid produces too much T4, you develop an overactive thyroid or hyperthyroidism. This can cause anxiety, difficulty sleeping, unexplained weight loss, heat intolerance, and rapid or irregular heartbeat.
The good news is that both conditions are treatable. We specialize in helping both men and women who have thyroid issues.
Testing Your T4 Levels
If you're concerned about your thyroid function, the first step is to get your hormone levels tested. Here’s what we’ll look at:
● TSH (Thyroid Stimulating Hormone): This pituitary hormone tells your thyroid to produce T4. It's usually the first test and works like a thermostat for your thyroid.
● Free T4: This measures the amount of T4 that's not bound to proteins and is available for your tissues to use.
● Total T4: This measures all T4 in your bloodstream, both free and protein-bound.
● Free T3: Sometimes measured to see how well your body is converting T4 to active T3.
● Thyroid antibodies: These help identify if an autoimmune process is affecting your thyroid.
Comprehensive thyroid testing gives a more complete picture of thyroid function than just checking TSH alone, which is why we prefer running a complete thyroid panel.
Supporting Healthy T4 Function
If you have a diagnosed thyroid condition, medication is usually necessary. For hypothyroidism, synthetic T4 (levothyroxine) is the standard treatment. Some patients benefit from combination therapy that includes both T4 and T3.
Beyond medication, you can support healthy thyroid function by:
● Getting adequate nutrition: Iodine, selenium, zinc, iron, and vitamins A, D, and B12 all support thyroid health and T4 production or conversion.
● Managing stress: Chronic stress (adrenal fatigue) can interfere with T4 conversion to T3 by increasing the production of reverse T3 (the inactive form).
● Limiting goitrogens: Some foods like soy, cruciferous vegetables, and millet can interfere with thyroid function when consumed in large amounts, especially if you're iodine deficient. These are called goitrogens.
● Addressing gut health: A healthy microbiome supports proper conversion of T4 to T3 in the gut.
● Avoiding environmental toxins (endocrine disruptors): Certain chemicals like perchlorates, BPA, and some pesticides can disrupt thyroid function.
If You’re Struggling with Hyperthyroidism or Hypothyroidism, We Can Help
If you're experiencing symptoms that might indicate a thyroid problem, don't ignore them. Hormone testing and thyroid treatment can make a tremendous difference in your overall health and quality of life.
At MedStudio, we offer comprehensive thyroid testing that looks beyond basic TSH to understand your complete thyroid picture.
Your next step is to get your hormone levels tested. Learn more about treatment options for thyroid disorders here.
You can also schedule an appointment with a specialist today to discuss getting your levels tested.