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Thyroid Stimulating Hormone TSH

Written by Ada’s Medical Knowledge Team

Updated on

TSH, meaning thyroid stimulating hormone, is an important hormone produced by the pituitary gland. The function of TSH is to let the thyroid know whether it has to produce more or less thyroid hormones. A higher level of TSH in the blood makes the thyroid produce more T3 and T4, while a lower level of TSH makes the thyroid produce less of these hormones. T3 and T4 both give feedback to the pituitary, so that it knows the amount of TSH that needs to be released in the blood.

As with any hormonal interaction, there can be a few issues with this system. Conditions of the thyroid or the pituitary gland can cause a level of TSH that is too high or too low. You can read all about TSH, its function and the conditions that may influence it in this article.

What is TSH?

TSH is a hormone that is produced by the anterior pituitary gland, which is a small gland right at the base of the brain. This gland secretes several hormones that regulate the production of other hormones in the body. TSH stimulates the thyroid gland to produce thyroid hormone. 1 2

The anterior pituitary knows when to secrete TSH because of a system called the hypothalamic-pituitary axis. This basically means that there is a strong interaction between three glands, namely the hypothalamus, the pituitary, and the thyroid gland. In this system, the hypothalamus secretes a hormone called TRH (Thyrotropin-Releasing Hormone, which can be referred to as the thyroid-releasing hormone), which causes the anterior pituitary to produce TSH. TSH then stimulates the thyroid to produce thyroid hormones. The main hormones that are produced by the thyroid are T3 and T4. Once there’s more T3 and T4 in the blood, this causes a reaction from the anterior pituitary, which will then decrease its production of TSH, otherwise known as a negative-feedback loop. 3

Why is TSH important?

TSH is important because it stimulates the production of thyroid hormones. These are very important hormones as they regulate crucial processes in our body. T3 and T4 have an effect on: 3 4

  • The regulation of the heart rate, respiratory rate and blood volume
  • The regulation of heat production
  • Oxygen delivery to the tissues
  • The promotion of skeletal development
  • An increase in wakefulness and alertness
  • The regulation of the reproductive system in men and women
  • The regulation of the gastro-intestinal function
  • The regulation of the pituitary function
  • Normal brain development - especially during pregnancy
  • Cell function and metabolism.

What are normal TSH levels?

Optimal TSH levels in adults should range between 0.45 – 4.5 mIU/L. mIU/L stands for milli-international units per liter. It’s a standard measurement agreed upon internationally by doctors and scientists. TSH is sensitive to any change in the plasma concentration of thyroid hormones. TSH may require an average of 6 to 8 weeks to adjust to changes in thyroid hormone levels. Therefore, it is recommended to check TSH levels 6 to 8 weeks after thyroxine adjustment or any antithyroid drug treatment. 5

For pregnant women, the normal TSH levels depend on which trimester they are in. 6 7

  • 1st trimester: 0.1 - 2.5 mIU/L
  • 2nd trimester: 0.2 - 3 mIU/L
  • 3rd trimester: 0.3 - 3 mIU/L

Thyroid function should be monitored closely in pregnant women with hypothyroidism, as thyroid replacement therapy with thyroxine may need to be increased. After delivery, thyroxine should be reduced to the pre-pregnancy dose and TSH should be rechecked after 6 weeks to further adjust thyroxine if needed.

During pregnancy, TSH should be monitored by measuring it every 4 weeks for up to 20 weeks gestation. After that, it should be tested at least once at 26 and 32 weeks gestation.

For children, there are other ranges for TSH levels as well. The normal TSH levels by age are: 6

  • 0 - 6 days: 0.7 - 15.2 mIU/L
  • 7 - 14 days: 0.72 - 11.0 mIU/L
  • 15 days - 16 years: 0.27 - 5.2 mIU/L

What is a TSH test?

A 3rd generation TSH test is a common test used to check your thyroid function. This test uses a blood sample to check the levels of TSH with high sensitivity. When the level of TSH is not within the normal range, the free T4 is also checked. This is T4 that is not bound to a protein in the blood. This test is also referred to as TSH with reflex to free T4. 8 9

What does high TSH mean?

A high level of TSH could indicate that your thyroid is underactive. This situation is referred to as primary thyroid disease. As there is a feedback loop between the thyroid and the anterior pituitary, the low amount of thyroid hormones in the blood cause the anterior pituitary to produce more TSH, so that the thyroid would be stimulated to produce more T3 and T4. The high level of TSH can also be caused by a problem with the pituitary gland. In that case, the condition is referred to as secondary thyroid disease. 3 8

An underactive thyroid is also called hypothyroidism. This is a condition that can have many causes: 10

  • Auto-immune diseases like Hashimoto’s thyroiditis. These are conditions where your own immune system attacks your thyroid.
  • Radioactive iodine treatment or removal of the thyroid to treat hyperthyroidism or thyroid cancer.
  • Intake of too much antithyroid medication that is used to treat hyperthyroidism.
  • Other medications that cause under-activity of the thyroid. These medications include lithium, amiodarone, interferon-alpha, tyrosine-kinase inhibitors, and immune checkpoint inhibitors such as nivolumab and pembrolizumab.
  • Malfunction of the pituitary gland.
  • Radiation therapy for cancers in the neck area.
  • Congenital hypothyroidism, which is a form of hypothyroidism that is present from birth.

What does low TSH mean?

A low level of TSH can be due to a problem with the thyroid, called primary thyroid disease. In this case, the thyroid will be overly active and will produce too much thyroid hormone. This condition is also called hyperthyroidism. Because of the negative-feedback loop to the anterior pituitary gland, the pituitary will produce less TSH as a reaction to the high levels of T3 and T4 in the blood. The low level of TSH can also be due to a problem with the pituitary, called secondary thyroid disease. 3

Hyperthyroidism can have many causes, such as: 11

  • An auto-immune disease called Graves’ disease
  • Thyroid nodules, which are lumps of thyroid cells that can produce thyroid hormone.
  • Thyroid goiter, which is an enlargement of the thyroid.
  • Thyroiditis, which is an inflammation of the thyroid gland.
  • Excess iodine intake
  • Too much thyroid hormone medicine
  • A non-cancerous tumor of the pituitary gland

How to get TSH levels balanced?

TSH levels can be balanced by treating hypothyroidism if you have high TSH levels, or by treating hyperthyroidism if your TSH levels are low. For hypothyroidism, treatment usually involves taking medication that replaces the missing thyroid hormones. The most frequently used medication is called levothyroxine. 12 13

In the case of hyperthyroidism, treatment can also include medication, which in this case is needed to prevent the thyroid from producing too much thyroid hormone. Radiotherapy or surgery may also be considered to remove the thyroid partially or completely. 14

In very rare cases, TSH levels outside their normal range can also be caused by a problem in the pituitary gland. This is often caused by a type of cancer that produces TSH. Treatment options for such tumors may include surgery or radiotherapy. 15


Q: How often to check TSH?

A: Current guidelines mention that people with thyroid issues should get a checkup every 6 to 12 months. When first starting treatment, the checkup can happen sooner to see how well the medication is working.

Q: How do you adjust levothyroxine if TSH is low?

A: Your doctor might recommend you to decrease your dosage of levothyroxine if your TSH is still low.

Q: How to increase TSH levels naturally?

A: Maintaining a healthy diet, proper stress management, having a healthy sleep rhythm and limiting caffeine and alcohol are all actions you can take to balance your TSH levels naturally.

Q: What is considered a dangerously high TSH level?

A: For an adult, a TSH level above 4.2 mIU/L is considered high and should be treated.

Q: What happens if TSH is over 100?

A: A high level of TSH is an indication that you have hypothyroidism. This can cause a number of symptoms such as tiredness, sensitivity to the cold and weight gain.

Q: What can cause a sudden increase in TSH levels?

A: Hypothyroidism can cause an increase in TSH levels.