Hyperthyroidism affects TSH levels, causing them to fall below expected ranges.

Hyperthyroidism can occur when the thyroid gland in the neck produces too much thyroid hormones, including triiodothyronine (T3) and thyroxine (T4).

These hormones are essential for regulating metabolism, heart rate, and energy levels. When levels are too high, a person can experience various nonspecific symptoms.

Doctors use measurements of thyroid hormones and thyroid stimulating hormone (TSH) to diagnose hyperthyroidism.

The pituitary gland in the brain produces TSH. TSH’s purpose is to regulate T3 and T4. TSH and thyroid levels have an inverse relationship, with low TSH suggesting hyperthyroidism.

This article examines hyperthyroidism TSH levels, typical ranges, and treatments for high or low levels.

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Generally, an abnormally low TSH level indicates hyperthyroidism.

The purpose of TSH is to stimulate the thyroid gland to produce thyroid hormones (T3 and T4). In hyperthyroidism, the thyroid gland is overactive and produces too much thyroid hormones. The high hormone levels trigger a feedback mechanism, and the pituitary gland reduces TSH secretion.

The expected range for TSH is 0.4–4.0 milli-international units per liter (mIU/L), according to the American Thyroid Association. Therefore, doctors may suspect hyperthyroidism for levels of 0–0.4 mIU/L. However, TSH ranges tend to increase with age. They also depend on pregnancy status.

Doctors cannot use TSH levels alone to make a diagnosis of hyperthyroidism. They usually conduct additional tests, including measuring T3 and T4 levels, to confirm the diagnosis. Testing also identifies the thyroid’s functional status and distinguishes hyperthyroidism from other thyroid-related conditions.

Learn more about hyperthyroidism.

Hyperthyroidism causes TSH levels to drop below the expected levels. This happens because the pituitary gland senses the increase in thyroid hormone levels in the blood and responds by reducing TSH production.

Hyperthyroidism has various effects on the body’s physiological systems, including:

  • Metabolic rate: Thyroid hormones are critical to metabolism. Elevated levels accelerate the rate. This means the cells consume energy more rapidly than usual.
  • Cardiovascular system: T3 and T4 significantly affect the heart and circulatory system. They increase heart rate and the force of heart contractions, leading to increased blood flow and oxygen delivery to the tissues. This causes a higher workload in the heart.
  • Thermoregulation: Thyroid hormones help regulate body temperature. In hyperthyroidism, the increased metabolic rate generates more heat, leading to increased body temperature.
  • Muscle and bone turnover: T3 and T4 influence muscle and bone turnover. In hyperthyroidism, the accelerated metabolic rate can increase the breakdown of muscle tissue and bone, affecting muscle strength and bone density.
  • Gastrointestinal function: An excess of thyroid hormones can speed up the movement of food through the digestive tract, affecting nutrient absorption and bowel movements.
  • Central nervous system: T3 and T4 affect the brain and nervous system. High levels can increase neural activity, affecting mood, behavior, and cognitive functions.
  • Reproductive system: Thyroid hormones influence reproductive health across sexes. Hyperthyroidism can disrupt normal reproductive function, affecting menstrual cycles and sperm production.

Signs and symptoms of hyperthyroidism include:

A person should contact a doctor if they experience symptoms of hyperthyroidism, especially if they are sudden or severe.

A doctor can assess the individual and rule out other conditions that could cause the symptoms. They can advise the person on treatment options. Early diagnosis and treatment can prevent complications.

Diagnosing hyperthyroidism typically involves a doctor:

  • reviewing a person’s symptoms and medical history
  • performing a physical examination
  • ordering blood tests to measure TSH, T3, and T4 levels

In some cases, doctors may order imaging tests, such as a scan of the thyroid gland or ultrasound.

Doctors use normal ranges to help guide their diagnostic and treatment processes. However, TSH levels can vary for several reasons, including:

  • the pituitary gland sends out TSH in pulses rather than a steady stream
  • labs not measuring every test exactly the same way
  • time of the test, as TSH levels normally increase at night and decrease during the day

By sex

The normal range of TSH is generally the same for males and females, typically between 0.4 and 4.0 mIU/L. However, some research suggests females have higher TSH levels.

Despite similarities in TSH levels between males and females, females have a higher risk of thyroid dysfunction than males, with 1 in 8 females experiencing problems at some point.

By age

TSH levels can increase as people age. Evidence suggests approximately 7–14% of older adults have TSH levels above the upper limit of reference ranges.

In children

TSH levels are usually highest at birth and gradually decrease with age. Therefore, normal ranges vary according to a child’s age.

There is very little information in the medical literature on TSH levels in children. However, 2012 research found the following data from 512 healthy children:

AgeReference ranges (mIU/L)
1 month1.18–3.57
1 year1.17–3.55
5 years1.15–3.47
12 years1.09–3.31
18 years1.05–3.16

Pregnancy increases the circulating levels of some thyroid hormones. This is critical for the brain and nervous system of the fetus. These changes result in decreased TSH levels.

Therefore, the reference ranges in pregnancy are lower. In the first trimester, the lower reference range for TSH shifts down by 0.4 mIU/L, while the upper reference range reduces by around 0.5 mIU/L.

There is a gradual return toward the nonpregnant range in the second and third trimesters.

However, there is significant geographic and ethnic diversity in TSH concentrations during pregnancy. So, doctors may use more localized reference ranges.

High TSH levels may indicate hypothyroidism. In this condition, the thyroid gland does not produce enough hormones. This prompts the pituitary gland to release more TSH to stimulate the thyroid.

Learn more about hypothyroidism.

Low TSH levels indicate hyperthyroidism, where the thyroid produces too much hormone, reducing the need for TSH.

Treating high TSH levels can involve thyroid hormone replacement therapy to normalize hormone levels.

The common approach is using the synthetic thyroid hormone levothyroxine. This medication replaces low T4 levels and restores hormone levels to a normal range.

Doctors carefully prescribe the levothyroxine dose based on a person’s weight, age, severity of hypothyroidism, and other health factors.

The individual will have regular thyroid function tests to allow doctors to monitor the condition and adjust the dosage appropriately.

Low TSH levels can indicate hyperthyroidism. Doctors may recommend treatment to reduce the overproduction of thyroid hormones.

Treatment options include medications such as methimazole and propylthiouracil to reduce the thyroid gland’s activity.

Other options include surgery to remove part or all of the thyroid gland or radioactive iodine therapy to destroy overactive thyroid cells.

A person with hyperthyroidism has low thyroid stimulating hormone (TSH) levels. TSH from the pituitary controls the release of thyroid hormone from the thyroid gland. Normal TSH levels are 0.4–4.0 mIU/L.

If a person has low TSH levels, treatment may involve antithyroid medications, radioactive iodine therapy, or surgery. In contrast, high levels require thyroid hormone replacement therapy.