The thyroid mainly produces thyroxine, called T4, and smaller amounts of triiodothyronine, called T3. T4 is the main hormone released into the blood, while T3 is the more active form used by cells. The thyroid also produces calcitonin, a hormone involved in calcium balance.
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See if you qualify →What hormones does the thyroid produce?
The thyroid gland produces two main thyroid hormones: thyroxine, also called T4, and triiodothyronine, also called T3. It also makes calcitonin, a separate hormone involved in calcium balance. T4 is the larger share of thyroid hormone released into the blood, while T3 is the more active hormone at the cell level 1.
T4, or thyroxine: the main hormone released by the thyroid
T4, or thyroxine, is the main hormone the thyroid releases into the bloodstream. The body can convert T4 into T3 in tissues such as the liver, kidney, and muscle, which lets different parts of the body adjust thyroid hormone activity based on local needs 1.
T3, or triiodothyronine: the more active thyroid hormone
T3, or triiodothyronine, is the more active thyroid hormone. It binds thyroid hormone receptors in cells and helps regulate gene activity that affects energy use, growth, heart function, and temperature control 2.
Calcitonin: a separate thyroid hormone involved in calcium balance
Calcitonin is made by special thyroid cells called C cells. It helps regulate calcium balance, but it is not the hormone people usually mean when they talk about “thyroid levels” or metabolism 3.
How does the thyroid know how much hormone to make?
The thyroid hormone feedback loop works like a thermostat. The brain senses thyroid hormone levels, then adjusts signals to the pituitary and thyroid so hormone levels stay in a healthy range over time 4.
The hypothalamus, pituitary, and thyroid feedback loop
The hypothalamus releases thyrotropin-releasing hormone, or TRH. TRH tells the pituitary gland to release thyroid-stimulating hormone, or TSH. TSH then tells the thyroid to make and release T4 and T3 4.
What TSH means on thyroid blood tests
TSH is not made by the thyroid. It is made by the pituitary gland, and it is often one of the first blood tests used to screen for thyroid problems. Clinicians often interpret TSH along with free T4, and sometimes T3 or thyroid antibodies, depending on the situation 5.
Why TSH often rises when thyroid hormone is low and falls when thyroid hormone is high
When thyroid hormone is low, the pituitary usually releases more TSH to push the thyroid harder. When thyroid hormone is high, TSH often drops because the pituitary is trying to slow thyroid output. This pattern is common, but not every case is simple, especially when pituitary disease, pregnancy, severe illness, or certain medicines are involved 5.
What does thyroid hormone do in the body?
T3 and T4 help set the pace for many body systems. They affect how cells use energy, how fast the heart beats, how the body handles temperature, and how the brain and body develop, especially during pregnancy and childhood 1.
Metabolism and energy use
Thyroid hormone helps regulate metabolism, meaning how the body turns food and stored fuel into energy. Too little thyroid hormone can slow this process, while too much can speed it up 1.
Heart rate and body temperature
Thyroid hormone affects the heart and blood vessels. Low levels may be linked with a slower heart rate and cold intolerance, while high levels may be linked with a fast or irregular heartbeat, heat intolerance, sweating, and tremor 6.
Growth, development, and brain function
Thyroid hormone is important for normal growth and brain development. During pregnancy and early life, untreated thyroid hormone problems can affect fetal and child development, which is one reason thyroid evaluation matters in pregnancy and postpartum care 7.
Menstrual cycles, mood, skin, hair, and bowel habits
Thyroid hormone can affect periods, mood, skin, hair, and bowel habits. Low thyroid hormone may be linked with constipation, dry skin, hair thinning, heavy or irregular periods, and low mood. High thyroid hormone may be linked with anxiety, sleep problems, lighter or irregular periods, and frequent bowel movements 1, 6.
What happens if thyroid hormone is low?
Hypothyroidism means the body does not have enough thyroid hormone for its needs. Common symptoms include fatigue, weight gain, cold intolerance, constipation, dry skin, hair thinning, heavy or irregular periods, and a slower heart rate, but these symptoms can have many causes 1.
Common symptoms of hypothyroidism
- Fatigue or feeling slowed down
- Weight gain or more difficulty losing weight
- Feeling cold when others are comfortable
- Constipation
- Dry skin or hair thinning
- Heavy or irregular menstrual bleeding
- Low mood or brain fog
- Slower heart rate
These symptoms are not specific to thyroid disease. Fatigue and appetite changes, for example, can come from sleep problems, anemia, depression, infection, medication effects, or hormone changes; our guide to fatigue and loss of appetite explains several common causes.
Common causes, including autoimmune thyroid disease and thyroid surgery
In the United States, Hashimoto’s thyroiditis, an autoimmune condition, is a common cause of hypothyroidism. Other causes include thyroid surgery, radioactive iodine treatment, some medicines, pituitary disease, and iodine deficiency or excess 1, 8.
How clinicians usually evaluate low thyroid hormone
Clinicians often start with TSH and free T4. Depending on the pattern and the person’s history, they may also check thyroid antibodies, T3, pregnancy status, iron, B12, or other labs. You can read more in our guide to treatment for thyroid dysfunction.
Treatment basics: levothyroxine and monitoring
Levothyroxine, a synthetic form of T4, is a standard treatment for hypothyroidism when medication is needed. Some research has studied levothyroxine alone versus levothyroxine plus liothyronine, a synthetic T3, in selected patients after thyroid surgery; in a 2025 feasibility double-blind trial, Phan and colleagues compared these approaches in postsurgical hypothyroidism 9. Thyroid hormone replacement therapy needs lab monitoring because too much thyroid hormone can raise risks such as fast heart rhythm and bone loss 10.
What happens if thyroid hormone is high?
Hyperthyroidism means the body has too much thyroid hormone activity. It can cause weight loss, rapid or irregular heartbeat, nervousness, heat intolerance, sweating, tremor, sleep problems, and frequent bowel movements 6.
Common symptoms of hyperthyroidism
- Fast, pounding, or irregular heartbeat
- Unexplained weight loss
- Feeling hot or sweating more than usual
- Tremor or shakiness
- Anxiety, irritability, or trouble sleeping
- More frequent bowel movements
- Muscle weakness
- Lighter or irregular periods
Possible causes, including Graves’ disease and thyroid nodules
Common causes include Graves’ disease, an autoimmune condition, and overactive thyroid nodules. Thyroid inflammation, excess iodine exposure, and taking too much thyroid hormone medication can also cause high thyroid hormone levels 6, 8.
Why high thyroid hormone can affect the heart
High thyroid hormone can increase heart rate and make some heart rhythm problems more likely, especially atrial fibrillation. This is one reason palpitations, chest pain, fainting, or shortness of breath should be evaluated promptly 6, 11.
Treatment depends on the cause and needs clinician evaluation
Treatment can include antithyroid medicines, radioactive iodine, surgery, or short-term symptom control while the cause is being worked up. Beta blockers may be used to help control symptoms such as rapid heart rate and tremor; Feely and Peden reviewed the use of beta-adrenoceptor blocking drugs in hyperthyroidism in 1984 12.
Can foods, iodine, or B12 affect thyroid hormone?
Iodine is needed to make thyroid hormone, but more is not always better. Too little or too much iodine can cause thyroid problems in some people, so high-iodine supplements and kelp products deserve caution 8.
Why iodine is needed to make T4 and T3
T4 and T3 are built from iodine and the amino acid tyrosine. T4 has four iodine atoms, and T3 has three. This is why iodine intake matters for thyroid hormone production 8.
Foods people often ask about: soy, cruciferous vegetables, kelp, and high-iodine supplements
Most people do not need to avoid soy or cruciferous vegetables such as broccoli, cabbage, or kale. The bigger concern is usually very high iodine intake from kelp or high-dose iodine supplements, especially in people with thyroid disease or thyroid nodules 8.
What B12 can and cannot do for thyroid hormone
Vitamin B12 does not replace thyroid hormone and does not make the thyroid produce T4 or T3. But low B12 can cause fatigue, weakness, numbness, balance problems, and mood or memory symptoms that may overlap with thyroid symptoms 13.
Why diet changes should not replace thyroid testing or prescribed treatment
Food choices can support general health, but they cannot confirm or rule out thyroid disease. If thyroid hormone is truly too low or too high, treatment decisions should be based on symptoms, exam findings, history, and lab testing.
When should you ask a clinician about thyroid testing?
Ask about testing if you have ongoing symptoms that fit low or high thyroid hormone, especially with risk factors. A typical first check includes TSH and free T4, but the right labs depend on your history 5.
Symptoms that may justify a thyroid check
- New or persistent fatigue, cold intolerance, constipation, dry skin, or hair thinning
- Unexplained weight change
- New palpitations, tremor, sweating, heat intolerance, or trouble sleeping
- Menstrual changes that are new for you
- Neck swelling, hoarseness, or trouble swallowing
- Symptoms after thyroid surgery, neck radiation, pregnancy, or a new medication
Risk factors such as autoimmune disease, pregnancy or postpartum status, family history, and age
Risk is higher in people with autoimmune disease, a family history of thyroid disease, prior thyroid surgery or neck radiation, pregnancy or postpartum status, and older age. Thyroid testing may also be important before or during certain medicines that can affect thyroid function 5, 7.
Why symptoms overlap with menopause, anemia, depression, medication effects, and other conditions
Symptoms can overlap across many conditions. For example, perimenopause can cause sleep changes, mood changes, cycle changes, and weight shifts; our guide to symptoms of perimenopause explains how hormone changes may show up. Testing helps avoid guessing.
Which next step fits your situation?
The right next step depends on symptoms, risk factors, and whether you already have abnormal labs. Thyroid testing is usually the bridge between “I have symptoms” and “I know what treatment, if any, makes sense.”
| Your situation | Sensible next step | Why it matters |
|---|---|---|
| You have fatigue, weight change, constipation, dry skin, or cold intolerance | Ask a primary care clinician about TSH and free T4 testing | These symptoms can fit hypothyroidism, but they also overlap with many other conditions. |
| You have palpitations, tremor, heat intolerance, sweating, or unexplained weight loss | Seek prompt medical evaluation, especially if symptoms are new or intense | High thyroid hormone can affect heart rhythm and needs clinician evaluation. |
| You are pregnant, trying to conceive, or recently postpartum | Ask an OB-GYN, primary care clinician, or endocrinologist about thyroid testing | Thyroid hormone needs can change during pregnancy and after delivery. |
| You already take levothyroxine or liothyronine | Do not change the dose on your own; ask for lab-guided follow-up | Too much or too little thyroid hormone can cause harm. |
| You are mainly worried about weight | Ask whether thyroid testing is appropriate, but avoid thyroid pills for weight loss without a thyroid diagnosis | Thyroid hormone is not a safe weight-loss shortcut; see our guide to thyroid pills for weight loss. |
Thyroid concerns and care at Chia
Chia does not currently offer thyroid hormone replacement therapy or treatment for hyperthyroidism. If your symptoms or labs suggest thyroid disease, a primary care clinician or endocrinologist is the right next step.
At Chia, our clinicians may consider thyroid history when reviewing eligibility for other care pathways, because thyroid disease can affect symptoms like weight change, fatigue, heart rate, mood, and menstrual patterns. Chia prescriptions for other treatments are available only after a licensed US provider reviews eligibility and determines treatment is clinically appropriate; a prescription is never guaranteed.
If you already have a thyroid diagnosis, our article on thyroid hormone replacement therapy explains the basics of levothyroxine, monitoring, and safety. If you are unsure where symptoms are coming from, start with your primary care clinician for testing before assuming the thyroid is the cause.
When should you get help now?
Get urgent care if symptoms suggest a serious heart, breathing, neurologic, or severe thyroid problem. Fast heartbeat with chest pain, fainting, confusion, or shortness of breath should not wait for a routine visit 6, 11.
- Chest pain, fainting, severe shortness of breath, or a very fast or irregular heartbeat
- Confusion, severe agitation, high fever, or severe weakness
- New neck swelling with trouble breathing or swallowing
- Severe symptoms during pregnancy or soon after delivery
- Symptoms after taking extra thyroid medication or high-dose iodine products
FAQ
The main hormone released by the thyroid is T4, also called thyroxine. The thyroid also makes smaller amounts of T3, the more active thyroid hormone, and calcitonin, which is involved in calcium balance.
No. TSH, or thyroid-stimulating hormone, is made by the pituitary gland in the brain. TSH tells the thyroid how much T4 and T3 to make.
Both matter. T4 is the main hormone released into the blood, while T3 is the more active form used by cells. Many tissues convert T4 into T3 as needed.
There is no universal list of five foods that everyone with thyroid problems must avoid. Most people do not need to avoid soy or cruciferous vegetables. Kelp and high-dose iodine supplements are more concerning because too much iodine can worsen some thyroid problems.
Vitamin B12 does not replace thyroid hormone or make the thyroid produce more T4 or T3. Low B12 can cause fatigue, weakness, numbness, and mood or memory symptoms that can overlap with thyroid symptoms.
Low thyroid hormone, called hypothyroidism, can cause fatigue, weight gain, cold intolerance, constipation, dry skin, hair thinning, heavy or irregular periods, low mood, and a slower heart rate. Testing is needed because these symptoms can have many causes.
High thyroid hormone, called hyperthyroidism, can cause weight loss, fast or irregular heartbeat, tremor, sweating, heat intolerance, anxiety, sleep problems, and frequent bowel movements. New palpitations, chest pain, fainting, or shortness of breath should be evaluated promptly.
Yes, thyroid hormone can affect weight because it helps regulate energy use. Low thyroid hormone can contribute to weight gain, and high thyroid hormone can cause weight loss. Thyroid medication should not be used for weight loss unless there is a diagnosed thyroid condition and a clinician is monitoring treatment.
References
- 1.National Institute of Diabetes and Digestive and Kidney Diseases. Hypothyroidism (Underactive Thyroid). NIDDK. 2021.
- 2.Mullur R, Liu YY, Brent GA. Thyroid hormone regulation of metabolism. Physiological Reviews. 2014.
- 3.National Cancer Institute. Calcitonin. NCI Dictionary of Cancer Terms. 2024.
- 4.Ortiga-Carvalho TM, Chiamolera MI, Pazos-Moura CC, Wondisford FE. Hypothalamus-pituitary-thyroid axis. Comprehensive Physiology. 2016.
- 5.National Institute of Diabetes and Digestive and Kidney Diseases. Thyroid Tests. NIDDK. 2021.
- 6.National Institute of Diabetes and Digestive and Kidney Diseases. Hyperthyroidism (Overactive Thyroid). NIDDK. 2021.
- 7.Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017.
- 8.National Institutes of Health Office of Dietary Supplements. Iodine Fact Sheet for Health Professionals. NIH. 2024.
- 9.Phan GQ, Yavuz S, Stamatouli AM, et al. A feasibility double-blind trial of levothyroxine vs. levothyroxine-liothyronine in postsurgical hypothyroidism. Frontiers in Endocrinology. 2025.
- 10.Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014.
- 11.Biondi B, Cooper DS. The clinical significance of subclinical thyroid dysfunction. Endocrine Reviews. 2008.
- 12.Feely J, Peden N. Use of beta-adrenoceptor blocking drugs in hyperthyroidism. Drugs. 1984.
- 13.National Institutes of Health Office of Dietary Supplements. Vitamin B12 Fact Sheet for Consumers. NIH. 2024.
About this article
Chia Health Editorial Team — Evidence-reviewed health education
This article is for educational purposes only and is not a substitute for individualized medical advice. Talk to a licensed clinician before starting, stopping, or changing any prescription.
AI tools may assist with research and drafting. Chia's editorial team reviews source use, clarity, treatment information, and safety framing before publication. A clinician is named only after explicit sign-off. Read our editorial standards.
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