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The thyroid is a butterfly-shaped gland located at the base of your neck, in front of your windpipe. Although you have many glands, the thyroid is especially important because it produces hormones that control metabolic processes throughout your body.
These metabolic processes depend on the thyroid sending out its hormones in just the right amounts. Too little or too much, and things begin to go awry. In this article, we explore the science behind the thyroid condition hypothyroidism (low thyroid), its causes and treatments, and how it differs from hyperthyroidism (high thyroid).
What Is the Thyroid Gland?
The thyroid is an endocrine gland, meaning it produces hormones that travel to, and have influence on, other parts of the body. There are hormones that regulate growth, stimulate development of the reproductive organs, and even control what happens when you feel stressed. (Source)
The Thyroid Hormones
The thyroid’s two primary hormones, triiodothyronine (T3) and thyroxine (T4), regulate metabolism — the complex chemical reactions that power all the work your body does. (Source)
The thyroid gland produces a third hormone, calcitonin, that reduces the amount of calcium in the blood if it gets too high. Although this is important for maintaining your body’s homeostasis, or equilibrium, calcitonin isn’t one of the hormones that concern us when we talk about hypo- and hyperthyroidism. (Source)
Hypothyroidism vs. Hyperthyroidism
As with many hormones, it is important that the concentration of thyroid hormones in your body stays within a certain range. When your thyroid gland does not produce the right amount of hormones, you may develop a thyroid condition. Hypothyroidism and hyperthyroidism are two thyroid conditions whose names sound very similar and are often confused.
Hypothyroidism is a condition in which your thyroid gland is underactive and is not producing enough thyroid hormones. On the other hand, hyperthyroidism is a condition in which your thyroid gland is overactive and is producing excess amounts of thyroid hormones.
Hypothyroidism is the more common of the two thyroid conditions, affecting around 3%–7% of the U.S. population. Hyperthyroidism, on the other hand, only affects around 1%–3% of people in the U.S. (Source, Source)
Either of these thyroid conditions can have symptoms and effects that range from mild to severe, depending on how underactive or overactive the thyroid gland is. The symptoms and treatments for hypothyroidism and hyperthyroidism can vary greatly, but either can go undiagnosed for a long time. (Source, Source)
The two thyroid disorders also have different causes, although both may result from autoimmune processes.
Causes of Hypothyroidism
The most common cause of hypothyroidism in developed countries such as the United States is Hashimoto’s thyroiditis, or Hashimoto’s disease. This is an autoimmune disorder in which the immune system attacks the thyroid gland, gradually destroying it. Hashimoto’s disease causes the thyroid gland to become inflamed and unable to produce sufficient thyroid hormones. (Source)
Hypothyroidism may also be caused by the use of lithium to treat certain psychiatric disorders, as lithium can affect the synthesis and secretion of thyroid hormones. Severe iodine deficiency can also lead to hypothyroidism because iodine is needed to synthesize thyroid hormones. Hypothyroidism due to iodine deficiency is very uncommon in the United States. (Source, Source)
Causes of Hyperthyroidism
Hyperthyroidism, on the other hand, is most commonly caused by Graves’ disease, another autoimmune condition. Normally, a hormone called thyroid stimulating hormone (TSH), released by the pituitary gland, binds to receptors on the thyroid and stimulates it to produce its thyroid hormones. Normally, the pituitary releases TSH in just the right amounts to keep thyroid levels within a certain range.
In Graves’ disease, though, antibodies bind to thyroid receptors in the same way TSH does, but they stimulate the thyroid in a way that is uncontrolled and causes it to overproduce its hormones. Like other autoimmune disorders, Graves’ is more common in women than in men. (Source)
Other causes of hyperthyroidism include solitary toxic adenomas, which are lumps that grow on the thyroid and produce thyroid hormones. Once common in iodine-deficient areas, there may be a genetic component to development of thyroid adenomas. On the other hand, iodine-induced thyroid dysfunction may be caused by ingesting excessive amounts of iodine. (Source, Source)
Treatment of Hypothyroidism vs. Hyperthyroidism
Thyroid hormone replacement therapy is commonly used to treat hypothyroidism.
The drug levothyroxine can be taken as thyroid hormone replacement therapy. Levothyroxine works by supplementing thyroid hormones in your body so your thyroid hormone levels are normal. The dosage of levothyroxine depends on age and health conditions. For example, women with hypothyroidism may need to adjust their levothyroxine dosage during pregnancy because of normal changes in thyroid function. (Source, Source)
There are three types of hyperthyroidism treatment.
Propylthiouracil and thiamazole are antithyroid drugs also known as thionamides (a third thionamide, carbimazole, is not used in the United States). These drugs work by targeting the thyroid gland and inhibiting thyroid hormone production. About 15% of patients taking thionamides experience minor side effects such as joint pain and itching, and while serious adverse reactions such as liver damage are rare, they do occur. (Source)
A thyroidectomy can be performed to remove either the entire thyroid gland or just part of it. This is the most successful treatment for Graves’ disease, and surgical side-effects are rare. If the entire thyroid gland is removed thyroid hormones will no longer be produced, and lifelong hormone replacement therapy will be necessary. After a partial thyroidectomy the remaining thyroid tissue will still produce some hormones, and hormone replacement may not be needed. (Source)
Radioactive Iodine Therapy
Radioactive iodine therapy is a type of radiation therapy used to destroy enough thyroid tissue so the patient is no longer producing excess thyroid hormones. This treatment type is recommended for patients who are not good candidates for surgery. Radioactive iodine therapy is generally safe and effective, although side effects do occur. Lifelong thyroid level monitoring is needed due to the possibility of permanent hypothyroidism. (Source, Source)
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Normal Thyroid Hormone Levels
“Normal” T4, T3, and TSH levels can vary based on age, the testing facility, and other factors such as pregnancy.
Free Thyroxine (T4)
Normal range, free T4: 0.7 ng/dL to 1.9 ng/dL (nanograms per decilitre)
Thyroxine, or T4, makes up about 95% of the thyroid hormones circulating in the blood. Most T4 is bound to proteins and unavailable for immediate use; this creates a pool of T4 that can be drawn from as needed. In lab tests, the protein-bound T4 is known as bound T4, whereas T4 that is not protein-bound is known as free T4, and together they make up total T4. However, free T4 tests are generally used to assess thyroid function. Several medications and medical conditions can affect total T4 levels. (Source)
Normal range, total T3: 80 ng/dL to 220 ng/dL
Triiodothyronine, or T3, is the active thyroid hormone that actually enters cells and affects their function. T3 is formed by removing an iodine atom from T4. Like T4, T3 is found in both bound and free forms. Free T3 tests can support a hyperthyroidism diagnosis. (Source)
Thyroid-Stimulating Hormone (TSH, thyrotropin)
Normal range: 0.35 mlU/mL to 4.50 mlU/mL
A high TSH may indicate an underactive thyroid (hypothyroidism), while a low TSH may indicate an overactive thyroid (hyperthyroidism). An abnormal TSH level with normal T3 and T4 levels may indicate a subclinical thyroid dysfunction — one that has not yet caused symptoms. (Source)
It may seem counterintuitive that TSH is elevated when the thyroid is underperforming, but this happens because the pituitary gland is registering low thyroid hormone output and trying to stimulate the thyroid to increase production. On the other hand, when the pituitary senses high thyroid hormone levels, it will decrease the amount of TSH it sends out, trying to lower thyroid hormone production to normal levels. (Source)
One Other Thyroid Test: Thyroid Antibodies
Another test that helps uncover thyroid disease is a thyroid antibodies test. Antibodies are proteins that your body produces to fight a virus or bacteria. However, when you have an autoimmune condition, antibodies mistake your body’s normal cells for foreign tissue and attack the cells. Hashimoto’s thyroiditis and Graves’ disease are both autoimmune diseases, and antithyroid antibodies may be detectable long before thyroid hormone levels become abnormal. Thyroid peroxidase and thyroglobulin antibodies are typically associated with Hashimoto’s disease, and TSH (also known as thyrotropin) receptor antibodies with Graves’. (Source, Source, Source)
Hypothyroidism in Men
Hypothyroidism is 5 to 10 times more common in women than in men. This is consistent with the fact that autoimmune diseases in general occur twice as often in women than in men. Despite this difference, the symptoms of hypothyroidism in men are nearly the same as those in women. (Source, Source)
In addition to the common hypothyroidism symptoms listed above, however, men with hypothyroidism may also experience:
In pregnant women, it’s important to consider the differences between subclinical and overt hypothyroidism, because each type can have a different effect on the pregnancy.
Subclinical hypothyroidism is defined as having an abnormal TSH level but normal T3 and T4 levels. This type is considered the less advanced form of hypothyroidism. Although there is evidence that subclinical hypothyroidism in the mother can lead to adverse pregnancy outcomes, there is also contradicting evidence, and more research must be done to confirm a causal relationship.
Overt hypothyroidism is the more advanced of the two and is defined as having both abnormal TSH and abnormal thyroid hormone levels. If untreated, overt hypothyroidism can cause complications in pregnant women such as preterm birth, preeclampsia, and miscarriage. There is also evidence that overt hypothyroidism can affect neurological and psychological development in the baby. However, you can lower your risk of developing these birth complications by taking a thyroid hormone replacement medication if needed. (Source, Source)
Hashimoto’s disease remains the most common cause of hypothyroidism in pregnant women. In general, if you are pregnant and receive a hypothyroidism diagnosis, it is important to consult your health care provider and monitor your TSH levels to minimize your risk of complications.
Is Hypothyroidism Genetic?
The short answer is maybe. Evidence suggests there’s a good chance that you and your parents have fairly similar thyroid hormone levels and similar likelihoods of developing a thyroid disorder. However, there is no evidence of a single gene that causes thyroid disorders. Autoimmune thyroid disorders are commonly present in multiple family members, but we do not have evidence that hypothyroidism is genetic. Read more about the link between your genetics and hypothyroidism in our article, Is hypothyroidism genetic? (Source)
Micronutrients and Hypothyroidism
Although there is no science-backed hypothyroidism diet, there are a few key micronutrients that may improve thyroid health. (Source, Source)
Iron plays an important role in the synthesis of thyroid hormones, and iron deficiency has been linked to decreased metabolism of T3 and T4. However, there is a lack of evidence that iron supplements or micronutrient supplements in general will affect the concentrations of T3 and T4.
Zinc is essential for maintaining thyroid function and for converting T4 to its active form. Like iron, zinc is important in the metabolism of thyroid hormones, but more research must be conducted to understand how these micronutrients affect thyroid hormone levels.
Selenium is a micronutrient that has a protective antioxidant effect on the thyroid gland and is needed for production of thyroid hormones. Excessive intake of selenium through unregulated supplements can be toxic, so instead try incorporating selenium-rich foods such as Brazil nuts into your diet. (Source, Source)
A Word About Iodine
Iodine is essential for production of thyroid hormones, and the primary cause of hypothyroidism in many countries is lack of iodine in the diet. The American diet, however, tends to include lots of foods that naturally contain iodine, as well as iodized salt and a variety of enriched food products. There is good evidence that consuming too much iodine actually contributes to thyroid autoimmunity, and so supplementing with iodine or intentionally seeking out foods that are high in iodine is generally not recommended. (Source)
AIP Diet and Hypothyroidism
Following a gluten free or autoimmune protocol (AIP) diet can reduce inflammation in the gut and help identify foods that cause an immune response. The AIP diet begins by eliminating inflammatory foods, then systematically reintroduces these foods to help identify which ones cause symptoms. A 2019 study found these diets can improve symptoms and reduce markers of inflammation in individuals with Hashimoto’s disease. However, there is no evidence that these diets improve thyroid function in people with Hashimoto’s. (Source, Source)
Should You Avoid Goitrogens?
Certain foods contain compounds called goitrogens, naturally occurring substances that may interfere with thyroid hormone production. Goitrogens are inactivated by cooking and the research suggests that, even raw, these foods are not likely to be a problem if you’re getting enough iodine in your diet.
The Bottom Line on Hypothyroidism vs. Hyperthyroidism
Currently, there is no scientific evidence hypothyroidism can be cured. As with other autoimmune diseases, you may be able to reduce some hypothyroidism symptoms through diet and lifestyle changes such as eating an anti-inflammatory diet and reducing stress. If your thyroid cannot produce adequate amounts of its hormones, though, supplementing with levothyroxine is currently the only way to maintain normal thyroid levels. If your hypothyroidism is autoimmune related, no evidence exists to suggest treatment changes the level of thyroid antibodies your body produces. (Source)
Hyperthyroidism, which is far less common than hypothyroidism, can be treated with medications that interfere with hormone production, or with invasive procedures that remove part or all of the thyroid gland. Trimming or removing the thyroid gland itself will reduce or eliminate production of thyroid hormones — a cure of sorts — but also means a lifetime of monitoring thyroid levels or replacing the hormones no longer being produced.