What Are The Causes Of Low TSH?

TSH is the hormone responsible for regulating the production of thyroid hormones. Your imbalance indicates problems with hypothyroidism and hyperthyroidism.
What are the causes of low TSH?

A low TSH level is indicative of an alteration that needs to be studied further. This exam is usually the first of several that will follow later.

TSH, or thyroid stimulating hormone, is a molecule that is involved in possible gland dysfunction. Therefore, when we have some symptoms related to this organ, the doctor orders a test that measures the levels of TSH in the blood.

What is TSH?

TSH is also called thyroid stimulating hormone or, technically, thyrotrophin. It is released from the pituitary after stimulation from the hypothalamus by another hormone known as thyrotrophin-releasing hormone, or TRH.

Once TSH is in the blood, it comes in contact with the cells of the thyroid gland to produce two more hormones:

  • T3 or triiodothyronine : it is much more active than T4 and it exerts effects on tissues, that is, it regulates its metabolism.
  • T4 or thyroxine : has minimal tissue activity. It is another T3 reservoir, because when T4 reaches the tissues, it can be converted into T3, which is the one that has the final action.

T3 and T4 will go to tissues to control the metabolism of proteins, carbohydrates and fats. Also, in babies, these hormones are very important because they support the normal development of the newborn’s brain tissue.

Low TSH is related to thyroid problems
The thyroid gland is stimulated by TSH, which in turn depends on TRH.

You May Be Interested: 4 Eating Habits to Improve Thyroid Health

Why can there be a low blood TSH level?

According to a study published in 2017, which included approximately 4,554 people aged between 18 and 93 years, the prevalence of hypothyroidism in the Spanish population was 9.1%. On the other hand, the prevalence of hyperthyroidism was 0.8%.

Together, these two disorders would affect nearly 10% of the population, and in all these patients it would be possible to find an abnormal TSH result. The hormone in question is regulated in 3 ways:

  • Amount of TRH released by the hypothalamus
  • The pituitary itself, which is responsible for storing and releasing TSH when needed
  • Negative feedback of thyroid hormones

What is negative feedback ?

When too many thyroid hormones are produced, they increase their amount in the blood. The hypothalamus and pituitary detect this increase and reduce the production of TRH and TSH. This is what we call negative feedback .

In other words, the hormones themselves self-regulate to have stable blood levels – and not so high. If this mechanism did not happen, we would constantly enter into hyperthyroidism or hypothyroidism.

Blood TSH Levels

Normal blood TSH levels are between 0.4 and 4 mIU/L. If there is an increase or a decrease in these values, it means that there is thyroid dysfunction.

In particular, a low TSH means there is so much T3 and T4 in the blood that they carry negative feedback to the pituitary, preventing production. Typically, in this scenario, we have the presence of hyperthyroidism.

There is also low TSH as a result of secondary hypothyroidism. This happens when there is a dysfunction in the pituitary, either because part of it has been destroyed or because something is preventing it from functioning properly, such as a brain tumor.

Lastly, tertiary hypothyroidism may be responsible. This means the hypothalamus does not work and stops producing HRT; therefore, it does not send signals for the pituitary to produce TSH.

In the first case, when there is an overproduction of peripheral thyroid hormones, we will find very high levels of T3 and T4 in the blood, indicating an excessive metabolism. The body will start burning all the fats and sugars in the body for energy.

Treatment of TSH Imbalances


The symptoms of hyperthyroidism are:

  • nervousness,
  • Palpitations,
  • Hyperactivity and increased sweating,
  • Hypersensitivity to heat,
  • Increased appetite and weight loss.

If you think you may have hyperthyroidism, see your doctor. The professional will request the blood dosage of TSH, T3 and T4. If TSH is low and thyroid hormones are elevated, the diagnosis of hyperthyroidism will be confirmed.

Causes can range from a thyroid tumor to an autoimmune disease in which the antibodies created are overexciting the thyroid cells. However, there are medications that reduce and control symptoms.

Thyroid control
Hyperthyroidism speeds up the heart rate and overall metabolism, causing weight loss.

Learn more: Thyroid nodules: symptoms and causes


Symptoms contrary to the above, such as fatigue, weight gain, cold intolerance and hypothermia, are indicative of hypothyroidism. We may have brain tumors that destroy the pituitary gland, or that compress it, preventing the release of TSH. There may also be a low production of T3 and T4 in the gland itself.

Low TSH in the context of hypothyroidism reveals a more serious problem, linked to pituitary destruction. Instead, if TSH is normal or high, suspicion is directed to the thyroid gland.

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