Low TSH: Causes, Symptoms, and How it Affects T3 and T4

Low TSH: Causes, Symptoms, and How it Affects T3 and T4
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Low TSH levels (thyroid-stimulating hormone) usually mean that your thyroid is producing too much T3 and T4 thyroid hormone. Because an overactive thyroid secretes too much thyroid hormones, signals are sent to your pituitary gland to secrete lower TSH levels. This can cause symptoms of low TSH or hyperthyroidism such as weight loss, nervousness, difficulty sleeping, or light periods (in women).

Hyperthyroidism isn’t just connected to low TSH levels. For doctors to diagnose and treat conditions related to an overactive thyroid, they also check levels of T3 and T4 in your blood. The causes of low TSH can be Graves’ disease, nodules that stimulate the thyroid, or inflammation.

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In this article, you will learn how low levels of TSH affect the thyroid hormones triiodothyronine (T3) and thyroxine (T4). I will also look at what should be the normal levels of TSH, T3, and T4 and how you can spot symptoms of hyperthyroidism.

What Does the Thyroid Do?

Your thyroid is a gland in the front of the neck that secretes important hormones in the body. This small, butterfly-shaped organ produces hormones that control metabolism and growth in the body.

Researchers from Johns Hopkins Medicine say that a healthy thyroid is needed to control how your body uses energy. Thyroid hormones affect your heart rate, body weight, menstrual cycles, intestines, and central nervous system. Hormones produced by the thyroid regulate how tissues produce or don’t produce proteins.1

What is TSH (Thyroid-Stimulating Hormone)?

Your pituitary gland in your brain produces a hormone called thyroid-stimulating hormone, or TSH for short.

Doctors from the American Association for Clinical Chemistry say that TSH affects levels of T3 and T4 hormones by stimulating the thyroid to produce these hormones. Checking for normal or low TSH levels can help to diagnose thyroid disorders.2  

For example, blood tests for TSH levels are used to routinely screen newborns for thyroid issues, monitor thyroid replacement therapy, or help diagnose infertility problems.

What is T4 and free T4 (FT4)?

Dr. Lisandro Irizarry on Medscape says that thyroxine is referred to as T4 and is the main hormone the thyroid produces. This is one of the most important hormones that the thyroid secretes. It contains 4 iodine molecules.3

Levels of free T4 (FT4) refer to the amount of thyroxine that is biologically active and not bound to protein. Doctors usually test for FT4 levels to help identify possible causes of thyroid issues. Free T4 is the active form of thyroxine.

What is T3 and free T3 (FT3)?

The thyroid hormone triiodothyronine is referred to as T3 because it contains one less iodine molecule than T4. Dr. Irizarry explains that thyroxine converts to triiodothyronine when it reaches the cells in the body. T3 is 4 times more active than the more abundant T4.3

As with FT4, free T3 (FT3) is the active level of thyroid hormone that can be used by the body.

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How TSH Affects T3 and T4 Levels

TSH affects levels of T3 and T4 in the bloodstream by stimulating the thyroid to produce these hormones. Doctors from the American Association for Clinical Chemistry say that the TSH mainly stimulates the production of T4 and this is converted into T3 by other tissues in the body.4

According to the journal Frontiers in Endocrinology, levels of T3 and T4 also affect how much TSH the pituitary gland produces. This is called a negative feedback loop. Higher levels of thyroid hormones (FT3 and FT4) in the blood signal to the pituitary gland to produce less thyroid-stimulating hormone (TSH).5

That is why low TSH levels indicate an overactive thyroid gland or hyperthyroidism.

Normal TSH Levels

What are normal TSH levels that indicate your thyroid is working properly and is in good health? According to the British Thyroid Foundation the normal range of TSH is as follows:21

  • 0.4 – 4.0 mIU/L (milliunits per liter)

However, recent studies into thyroid activity and what should be classed as normal TSH levels put the upper range somewhat lower. Researchers in the Journal of Clinical Endocrinology & Metabolism recommend that the upper TSH limit should be 2.5 mIU/L.6

This is because many doctors say that individuals with TSH readings between 2.5 mIU/L and 4.0 mIU/L are at risk of developing thyroid disorders.

It should also be remembered that TSH levels are higher in older individuals. For example, the upper range for persons between 50 and 59 is 4.5 mIU/L and in persons over 80, TSH levels up to 7.5 mIU/L would still be considered normal.6

What are Low TSH Levels?

The normal reference range for TSH blood tests is between 0.4 and 4.0 mIU/L. Therefore, if your blood test readings for TSH show up lower than 0.4, you have low TSH levels. Doctors will then run further tests to check free T4 and possible free T3 in blood serum.

What Does it Mean When Your TSH is Low

If your TSH levels are low it could mean that you have hyperthyroidism because your thyroid is probably producing excess amounts of T3 and T4 hormones. Some other disorders that are not thyroid related can show low TSH and also low or normal FT3 and FT4.

Let’s look in more detail at reasons for showing signs of hyperthyroidism.

Graves’ disease

Graves’ disease is the most common cause of hyperthyroidism where you have low TSH and elevated T4 and or T3.

The American Thyroid Association say that Graves’ disease is an autoimmune condition that creates antibodies which put the thyroid gland into working overtime. This results in an increase of thyroid hormones T4 and T3 and therefore your pituitary gland produces less TSH.6

Some of the symptoms of Graves’ disease include:

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  • Swelling around the eyes giving them a bulging appearance
  • Inflammation of the eyes making them appear red
  • Vision problems
  • Lumpy reddish patches of skin (a rare symptom of Graves’ disease)

Plummer’s disease

Plummer’s disease is a condition where multiple nodules become overactive in the thyroid gland, causing it to secrete more thyroid hormones than it should.

According to Dr. Philip R. Orlander, who is a professor of endocrinology, Plummer’s disease is also called toxic nodular goiter (TNG). In many cases, the nodules start secreting T3 and T4 hormone independently of the thyroid and they can become toxic.7

Other symptoms of TNG or Plummer’s disease can include:8

  • Unintended weight loss especially in elderly individuals
  • Frequent bowel movements
  • Hand tremors
  • Heart problems
  • Swelling around the neck (enlarged goiter)

Nodules or cysts in the thyroid gland

Other types of growths in the thyroid can also stimulate the release of more than normal thyroid hormone levels, thus cause TSH levels to drop significantly.

According to an expert in endocrinology, Dr. Robert Ferry Jr., thyroid nodules or cysts may or may not affect thyroid function. In some cases, the benign growths in the thyroid can stimulate hormone production causing hyperthyroidism. In other cases, the growths can cause an underactive thyroid.9

Some signs of thyroid cysts include:

  • Pain that spreads from the front of the neck to the ear or jaw
  • Visibly enlarged lump in the throat
  • Difficulty swallowing
  • Low TSH levels

Excess iodine

Proper levels of iodine are essential for the health of the thyroid gland. If you have too much iodine in your diet, your TSH levels could drop and you may start showing sign of an overactive thyroid gland.

The journal Nature Reviews Endocrinology reports that iodine is naturally found in kelp, bread, milk, some vitamin supplements, and salt fortified with iodine. The recommended amount of iodine needed per day for proper thyroid function is 150 μg.

Researchers say that, in healthy persons, it is rare to overdose on iodine from a healthy diet. However, people with autoimmune conditions may be more at risk from excess iodine exposure from their diet.10

In cases of underactive thyroid, doctors may recommend a diet rich in foods containing iodine for hypothyroidism.

Inflammation of the thyroid (thyroiditis)

If your TSH levels are low, it could mean that you have a condition called thyroiditis. Inflammation of the thyroid can cause your thyroid to swell and start secreting more T4 and T3 than it should. This increase of thyroid hormone results in depleted TSH levels.

According to researchers from the Cleveland Clinic, there are many reasons why the thyroid can become inflamed. Low TSH caused by thyroiditis could be the result of a viral or bacterial infection, radiation therapy, or antibodies that attack the thyroid gland.11

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Thyroiditis can result in either hyperthyroidism or hypothyroidism depending on how the inflammation affects the thyroid.

Tumors of ovaries or testes

Tumors in the ovaries or testes affect your hormones and can suppress TSH levels. When this happens you will show signs of hyperthyroidism.

OB/GYN Dr. Lisa Rubinsak from the Emory University School of Medicine says that a type of ovarian cancer called struma ovarii can cause hyperthyroidism. This is a rare type of cancer, and hyperthyroidism is a symptom in 5-8% of patients with struma ovarii. The typical symptoms of ovarian cancer include:12

Testicular cancer in men can also result in symptoms of hyperthyroidism. Dr. Kush Sachdeva who specializes in oncology says that cancer of the testes causes thyroid hormones to rise in the body. Other symptoms of testicular cancer can include:13

Benign tumors of the thyroid gland or pituitary gland

Non-cancerous growth in the thyroid gland or pituitary gland can mean that you have low thyroid-stimulating hormone in your body.

Doctors from the Mayo Clinic say that most small growths in the thyroid are not serious and don’t show any symptoms. If the growths become large, they can affect the production of thyroid hormones and suppress TSH levels. In these cases, you will show symptoms of hyperthyroidism.14  

Drug-induced thyroid disorders

Low TSH symptoms can manifest themselves if you are taking certain medications. According to information published in the journal American Family Physician, treatments for an underactive thyroid gland can cause low TSH levels if the dosage is too high. Also, some drugs for an irregular heartbeat or medication containing iodine can also cause symptoms of low TSH.15

Thyroid cancer

On rare occasions, growths in the thyroid can become cancerous and the thyroid needs to be removed to prevent the cancer spreading.

The American Cancer Society reports that only about 2 in 20 cases of thyroid nodules turn out to be malignant. If the tumor stimulates the thyroid, the result will be hyperthyroidism. Doctors say that growths in the thyroid that cause low TSH and elevated T4 are almost always benign.16

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What Are the Effects of Low TSH Levels?

Symptoms of low TSH levels are usually associated with hyperthyroidism. Dr. Oliver Starr on Patient.info says that having too much thyroxine in the blood will cause the pituitary gland to secrete less thyroid-stimulating hormone.17

Having low TSH levels usually means that you will have one or more of the following symptoms:

  • Feeling nervous, irritable, or finding it difficult to relax
  • Disrupted sleep patterns and/or insomnia
  • Heart palpitations and an increased heart rate
  • Increased sweating
  • General fatigue and muscle weakness
  • Diarrhea or passing stools more often than you used to
  • A visibly larger neck
  • Bulging eyes

It’s important to get the appropriate treatments for signs of hyperthyroidism because, if left untreated, you can have any of the following complications:

  • Increased risk of developing osteoporosis
  • Heart disease, for example, an increased risk of angina, irregular heartbeat, or even heart failure
  • Complications during pregnancy

Low TSH but Normal Free T4

What does it mean if blood tests show low TSH level but the free T4 levels are normal?

According to the book Screening and Treatment of Subclinical Hypothyroidism or Hyperthyroidism, low TSH but FT4 within the normal range could indicate that you have subclinical hyperthyroidism. Doctors generally recommend anti-thyroid treatment if blood tests show TSH levels below 0.1 mIU/L. Low TSH and normal FT4 is common in people who have Graves’ disease or nodular thyroid disease.18

Another reason for low TSH levels and normal free T4 is if you are undergoing T4 treatment for hypothyroidism.

Low TSH but Normal T3 and T4

Why can you have low TSH levels but normal T3 and T4?

The reasons for T3 and T4 being within the normal reference range but having TSH lower than 0.1 mIU/L can also be subclinical hyperthyroidism. The journal British Medical Bulletin reported that common reasons for low TSH serum level and normal T3 and T4 can include:19

  • Levothyroxine treatment
  • Graves’ disease
  • Viral thyroiditis
  • Toxic nodular goiter in elderly patients

Low TSH and High T4

Blood test results for thyroid disorders that show low TSH and high thyroxine levels usually confirm hyperthyroidism. Dr. Oliver Starr on Patient.info says that in these circumstances, doctors will prescribe treatment to lower levels of thyroxine in the blood which should then increase TSH levels.17

Low TSH, Low T4 and Low T3

In cases where blood tests show that you have low levels of both thyroid-stimulating hormone and thyroxine, the cause is usually not connected with your thyroid.

According to endocrinologist Dr. Serhat Aytug, conditions causing low TSH and low T4 are classed and non-thyroidal illness syndrome. This is usually seen in persons who are critically ill or who have a severe chronic disease. Some examples of chronic disease that can cause T4, T3, and TSH to drop below the normal reference range are:20

  • Any severe disease affecting the gastrointestinal tract, the lungs, heart, or liver
  • Heart attack
  • Starvation
  • Severe burns
  • Bone marrow transplant

Read my other related articles:

Medical Sources

  1. HopkinsMedicine. Thyroid gland.
  2. LabsTestsOnline. Thyroid-stimulating hormone.
  3. Medscape. Thyroid hormone toxicity.
  4. LabsTestsOnline. Free T3 and Total T3.
  5. Front Endocrinol (Lausanne). 2015; 6: 177.
  6. ThyroidOrg. Graves’ disease.
  7. Medscape. Toxic nodular goiter.
  8. Medscape. Toxic nodular goiter clinical presentation.
  9. MedicineNet. Thyroid nodules.
  10. Nat Rev Endocrinol. 2014 Mar; 10(3): 136–142.
  11. ClevelandClinic. Thyroiditis.
  12. Medscape. Struma ovarii clinical presentation.
  13. Medscape. Testicular cancer clinical presentation.
  14. MayoClinic. Thyroid nodules.
  15. Am Fam Physician. 2005 Aug 15;72(4):623-630.
  16. CancerOrg. What is thyroid cancer?
  17. PatientInfo. Overactive thyroid gland.
  18. NCBI. Screening and treatment of subclinical hypothyroidism or hyperthyroidism.
  19. Br Med Bullet. 2013 Sept;107(1): 101-116.
  20. Medscape. Euthyroid sick syndrome.
  21. BTFThyroid. Thyroid function tests.
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