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TSH 101: What low and high TSH levels actually mean

TSH 101: What low and high TSH levels actually mean

10 min read

One in eight people with ovaries will develop a thyroid disorder during their lifetime. And the real kicker is that an estimated 60% of people with thyroid disorders are unaware of their condition. But there's one great way to peek under the hood of your thyroid to understand more about it: testing thyroid-stimulating hormone (TSH), which stimulates (hence the name) the thyroid gland to produce thyroid hormones.

“Thyroid hormones are important for metabolism, general health, your sense of well-being, and many, many processes," explains Modern Fertility Chief Medical Advisor and reproductive endocrinologist Dr. Nataki Douglas, MD, PhD. "But in pregnancy, thyroid hormones are also important for the health — especially the neurological development and cognitive functioning — of your developing baby.”

Here are the key things to know about TSH:

  • TSH is a messenger hormone that tells your thyroid gland to increase or slow down production of thyroid hormones, which help regulate the pace for your body’s energy usage.
  • Having too much thyroid hormone (hyperthyroidism) or too little (hypothyroidism) can have serious health implications if not treated. Testing your TSH value can help you understand how your thyroid is functioning.
  • TSH is known to be a somewhat volatile hormone, and it’s not unexpected to see TSH levels vary between tests. Your doctor can help you interpret your results.
  • Your thyroid and your fertility are connected. Testing your TSH can tip you off to thyroid conditions that can impact your ability to get and stay pregnant.
  • It’s easy to test your TSH level at home with the Modern Fertility Hormone Test.

What is TSH and why does it matter?

Just as the name suggests, thyroid-stimulating hormone (TSH) is a hormone that helps regulate your thyroid gland. Your thyroid is part of the body’s endocrine system, a network of glands that communicate with each other and cells and organs to keep things running smoothly. You can think of the glands in the endocrine system kind of like co-workers with specific areas of expertise.

TSH is produced by the pituitary gland, a smaller-than-pea-sized gland located in the base of the brain that’s sometimes referred to as the “master gland” because it tells other glands what to do (basically, the pituitary gland = the boss). One very important gland the pituitary gives marching orders to? Our friend, the thyroid gland — a butterfly-shaped gland that lives above the hollow of your neck.  

What does TSH do?

TSH is a “messenger hormone” that tells the thyroid gland to produce thyroid hormones. There are two main forms of thyroid hormone: triiodothyronine (T3) and thyroxine (T4). They’re primarily responsible for regulating your metabolic rate, which is how quickly your body uses energy. T4 is produced in much higher concentrations than T3 (about 95% of the thyroid hormone your body produces is T4). Because the amount of TSH your body produces is directly linked to how much thyroid hormone you produce, TSH is a good measure of thyroid gland function.

How does your body know when to produce TSH? Your pituitary gland works in a feedback loop with your thyroid to make sure there are the right amount of thyroid hormones circulating in your bloodstream. The pituitary keeps tabs on the levels of T3 and T4 in the bloodstream, and ramps up or slows down production of TSH accordingly:

  • When the pituitary senses that levels of thyroid hormones in the bloodstream are low, it ramps up production of TSH.
  • When the pituitary senses that levels of thyroid hormones in the bloodstream are high, it slows production of TSH.

What does it mean to have low TSH levels?

Low TSH levels are an indicator of an overactive thyroid gland. Low TSH happens when the pituitary gland notices that there’s an excess of thyroid hormones (T3 and T4) in the bloodstream and cuts down on the amount of TSH it’s sending to the thyroid. Medically, this condition is known as “hyperthyroidism.” Hyperthyroidism is diagnosed when TSH is below 0.4 uIU/mL on more than one test (or multiple tests) taken in a short period of time.

The thyroid is involved in setting the pace for different body systems and controlling the allocation of energy (aka your metabolism). If there’s an excess of thyroid hormones in your system, the pace of these systems will accelerate.

What are symptoms of low TSH and hyperthyroidism?

Common symptoms of hyperthyroidism include:

  • Nervousness
  • Heart racing
  • Hand tremors
  • Anxiety
  • Difficulty sleeping
  • Weight loss
  • Fatigue
  • Thinning of skin
  • Brittle hair
  • Muscle weakness
  • Infrequent periods

What can cause low TSH and hyperthyroidism?

Here are some of the causes and types of hyperthyroidism:

  • Graves' disease: In over 70% of cases, an overactive thyroid is caused by Graves' disease. Grave’s disease is an autoimmune disorder in which specific antibodies produced by the immune system trigger cells in the thyroid to go into overdrive on the production of thyroid hormone. It tends to run in families and is most common in young people with ovaries.
  • Toxic nodular or multinodular goiter: A type of hyperthyroidism that happens when additional nodes grow on the thyroid, causing the overall production of thyroid hormone to increase
  • Thyroiditis: Thyroiditis is inflammation of the thyroid gland. It can lead to both under- and overproduction of thyroid hormones.
  • Too much synthetic thyroid hormone: Taking too much synthetic thyroid hormone can also result in thyroid hormone levels that are too high.

How is hyperthyroidism treated?

There are a variety of treatments for hyperthyroidism. Your doctor will consider your age, the severity and type of hyperthyroidism you have, any other medical conditions, and your preference for treatment when creating a plan. Some of the ways that hyperthyroidism is treated include:

  • Antithyroid medications to decrease the thyroid’s ability to produce thyroid hormone.
  • Surgical removal of thyroid tissues to shrink or reduce the number of thyroid hormone-producing cells.
  • Radioactive iodine, also to shrink or reduce thyroid hormone-producing cells.
  • Beta blockers in conjunction with the above therapies to reduce the symptoms associated with hyperthyroidism, including nervousness and heart racing.

What does it mean to have high TSH levels?

On the flip side, high TSH levels point to an underactive thyroid. Why? When the pituitary gland senses that thyroid hormone levels (T3 and T4) in the blood are low, it produces a lot of TSH to try to get the thyroid to kick thyroid production into gear. The term for the diagnosis of underactive thyroid is “hypothyroidism.” In general, if your thyroid gland isn’t producing enough thyroid hormones, the pace of body systems will slow down. Hypothyroidism is diagnosed when TSH is above 4.0 uIU/mL on more than one test (or multiple tests) taken in a short period of time.

Physicians distinguish between “overt” hypothyroidism (when high TSH and low t4 levels clearly point to hypothyroidism and treatment is necessary) and “subclinical” hypothyroidism (when elevated TSH is the only out-of-range hormone). There is some disagreement in the medical community around whether treatment is needed for subclinical hypothyroidism.

What are symptoms of high TSH and hypothyroidism?

Common symptoms of hypothyroidism include:

  • Dry skin
  • Cold intolerance
  • Fatigue
  • Forgetfulness
  • Hair loss
  • Weight gain
  • Constipation

Remember how at the top of this article we mentioned that 60% of people with a thyroid disorder are unaware of the condition? Part of the reason thyroid disease is under-diagnosed is because symptoms are somewhat “non-specific.” It’s important to chat with your healthcare provider if you think you may have a thyroid condition or if you have a family history of them.

What can cause high TSH and hypothyroidism?

There are many reasons your thyroid gland might not be making enough thyroid hormone. Here are some of the most common causes:

  • Autoimmune diseases (like Hashimoto’s Disease): Autoimmune diseases can cause your body to mistakenly attack cells in the thyroid, leaving fewer cells than normal to produce the needed amounts of thyroid hormone.
  • Surgical removal of part (or all) of the thyroid gland or radiation: Sometimes in order to treat an overactive thyroid, a doctor will surgically remove some or all of tissues making up the thyroid gland, or treat the gland with radiation to reduce the number of cells producing thyroid hormone. Depending on the amount of thyroid tissue remaining after these procedures, your body might make too little thyroid hormone.
  • Being born without a thyroid gland or with a partially formed thyroid: If you are born without a fully functioning thyroid, you’ll produce less thyroid hormone than is needed.
  • Thyroiditis: Like we mentioned earlier, inflammation of the thyroid can both decrease and increase thyroid hormones.
  • Other factors: Certain medications, iodine imbalance, damage to the pituitary gland, and rare disorders that impact thyroid function can also play a role.

How is hypothyroidism treated?

Hypothyroidism can be managed by taking synthetic thyroxine (under the supervision of a doctor) to replace the thyroid hormone your thyroid gland isn't producing and bring thyroid levels up to the optimal level in your body. Hypothyroidism is a manageable condition, but you’ll need to continue taking medication and monitoring your levels throughout your life.

What else can cause low or high TSH?

TSH levels can be impacted by illness (acute or chronic), pregnancy, medications, food or supplements, and even a lack of sleep. There are also fluctuations in TSH throughout the day: TSH tends to be higher in the mornings and decreases throughout the day.

The small variations we just mentioned mean that occasionally TSH levels tested some time apart don't match up. But even accounting for factors that can cause small amounts of variation between tests, TSH results are an important piece of insight about what’s going on with your thyroid gland. If you get two TSH results that don’t match up, for example one is in the “high” range and the other is in the “normal" range, it’s possible that in the first test your level was temporarily elevated — it doesn’t necessarily mean that one of the test results is wrong.

If you’ve done multiple TSH tests and the results seem discordant, talk with your doctor. They can look at the two results in context of the reference ranges from the lab where you took your sample and help you interpret them. They may also order some additional testing to check other aspects of thyroid function.

What does your TSH level have to do with your fertility?

Thyroid dysfunction can impact the whole body, including your reproductive system. Testing (or keeping tabs) on your TSH can provide early warning signs that you may have a thyroid problem that’s interfering with your ability to get or stay pregnant.

One of the major intersections between TSH levels and fertility? Thyroid dysfunction can disrupt your menstrual cycle. Out-of-range thyroid hormones can lead to heavy or irregular periods, or to not getting a period for several months or longer (a condition called amenorrhea).

Irregular periods can mean that you’re not ovulating as regularly as you otherwise would be. If you’re trying to conceive, this makes the window of time when you can get pregnant come around less often — and in a more unpredictable timeframe. In addition to making your whole cycle more irregular, thyroid dysfunction can also interfere with the hormones that jumpstart ovulation. For example, some people who have hypothyroidism produce an excess of prolactin (the hormone that stimulates production of breast/chest milk), and this can block ovulation.

As we’ve discussed on the blog before, irregular or absent menstruation can tip you off to underlying health conditions. If you experience cycle irregularity, make sure to discuss it with your healthcare provider — whether or not you’re concerned about a thyroid condition.

TSH and pregnancy

If not managed, thyroid conditions can also contribute to pregnancy complications that impact both the pregnant person and developing fetus.

Untreated hyperthyroidism during pregnancy can lead to:

  • Increased risk of premature birth
  • Increased risk of preeclampsia
  • Increased risk of thyroid storm (sudden, severe worsening of symptoms)
  • Increased risk of a fast heart rate in the newborn
  • Increased risk of low birth weight
  • Increased risk of miscarriage

Untreated hypothyroidism during pregnancy can lead to:

  • Increased risk of anemia
  • Increased risk of preeclampsia
  • Increased risk of low birth weight
  • Increased risk of miscarriage
  • Increased risk of stillbirth
  • Problems with fetal growth and brain development

Because of the links between thyroid function and pregnancy outcomes, guidelines suggest that people should achieve a TSH level below 2.5 uIU/mL during the first trimester. The thing is, doctors don't recommend testing for TSH proactively — and there are different perspectives among providers on the exact cutoff that warrants follow-up testing and treatment before becoming pregnant or while trying to conceive.

While infertility evaluations will typically include TSH testing, "in primary care and GYN practices, women are not being screened if they do not endorse symptoms of low/high thyroid hormone levels," says Dr. Douglas. (Because thyroid issues aren't routinely screened for, and they can cause issues with fertility or pregnancy, we make checking in with your TSH more accessible with the at-home Modern Fertility Hormone Test.)

Despite no proactive TSH testing, even subclinical hypothyroidism can be risky during pregnancy. Research supports treatment of subclinical hypothyroidism and significantly elevated TSH (above 4.2 uIU/mL) in pregnant people to decrease the rates of miscarriage and increase the rate of live births. Additionally, untreated subclinical hypothyroidism has been associated with adverse neurodevelopmental outcomes in kids. But not all subclinical hypothyroidism will be treated preconception.

As Dr. Douglas explains it, treatment may only be recommended for people who are trying to get pregnant if they also have anti-thyroid antibodies (which are associated with pregnancy complications). Treatment for subclinical hyperthyroidism, on the other hand, is not typically recommended because it hasn't been linked with adverse pregnancy outcomes.

How can you measure your TSH levels?

If you’re concerned about your thyroid or think you might have a thyroid condition, you can always check in with your healthcare provider about what lab tests they may recommend.

Another easy way to get a read on your TSH is with the Modern Fertility Hormone Test. You’ll order a kit from our site, take a simple finger-prick blood test at home (or swing by a lab if you prefer), and have results uploaded in a personalized dashboard within 10 days. Depending on your TSH result from the first round of testing, our clinical team may issue a free retest for both TSH and free T4 to get more insight into your thyroid function. (Note: If your TSH level is extremely high or low, your results dashboard will include a recommendation that you follow up directly with your doctor instead to get treatment ASAP.)

Hannah had been trying to conceive for a few months before deciding to take the Modern Fertility Hormone Test — which provided "peace of mind" as well as "confirmation." "I have been struggling with thyroid symptoms for years and medical professionals have not taken me seriously. The blood work from the Modern Fertility test revealed that I have high levels of TSH and I am finally on the path toward treatment," Hannah says. "I encourage everyone to take the Modern Fertility test. If you are planning to get pregnant or are trying, it may provide more insight into your health than you realize. Thanks to Modern Fertility, I am starting to feel normal again.”

The bottom line on TSH

The thyroid gland is a key player in helping your whole body run smoothly, and checking your TSH level is a powerful way to keep tabs on it. When the thyroid gland isn’t producing the right amount of thyroid hormones, the whole body can be impacted — including fertility.

Thyroid conditions can be managed by working closely with your doctor, but getting a diagnosis is the first step. While the Modern Fertility Hormone Test alone can’t diagnose a thyroid condition, having a read on your TSH level can open up the door to conversations with your healthcare provider. For thyroid knowledge on your terms, order a Modern Fertility Hormone Test here.

This article was medically reviewed by Dr. Nataki Douglas, MD, PhD, the Chair of the Modern Fertility Medical Advisory Board.

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Jen Lehr

Jen is a member of Modern Fertility's content team. She's a distance runner, outdoor adventure enthusiast, and humbly claims to have mastered the art and science of snacking.

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