What Does it Mean When Your TSH is Low?

TSH is a hormone made by the pituitary gland that tells the thyroid gland how much thyroid hormone to make. Symptoms of low TSH include weight loss, fatigue, confusion, heat intolerance, fast heartbeat, high blood pressure and other symptoms.

The endocrine system is responsible for regulating the function of organs in your body. The pituitary gland produces thyroid stimulating hormone (TSH). TSH stimulates the thyroid gland to produce thyroid hormones.

If the pituitary gland is not functioning to produce TSH as it should, then the thyroid will not make the level of hormones it should be. This reduction of hormones from the thyroid can lead to many health problems.

Low TSH levels can affect your health, quality of life, and your relationships. The signs of the conditions caused by a low TSH level are essential to know so that you can recognize that you might have a health issue and discuss it with your doctor to get the proper treatment.

Thyroid hormones act on a feedback loop. The pituitary gland is signaled to stop producing TSH when there is enough thyroid hormones, like thyroxine (T4) and triiodothyronine (T3), in the blood. When something other than the pituitary gland causes excess thyroid hormone production, TSH levels will drop.

The conditions associated with low TSH levels produce a lot of symptoms, including:

• Weight loss
• Sleep issues
• Bulging eyes
• Light sensitivity
• Fatigue
• Irregular menstrual cycles
• Confusion
• Lack of heat tolerance
• Weak muscles
• Shaky extremities
• Fast heart rate
• Excess sweating
• Irritability
• High blood pressure
• Brittle hair
• Diarrhea and frequent bowel movements

Low TSH levels can generally manifest as four types of thyroid conditions:

• Primary hyperthyroidism, which occurs when you have high thyroid hormone levels and low TSH levels.
• Mild hyperthyroidism is a thyroid condition that results in a low TSH level and normal thyroid hormone levels.
• Thyroiditis is an inflammation of the thyroid because of other conditions.
• Pituitary disease, which causes the pituitary gland to not produce enough TSH to properly stimulate the thyroid gland.

Autoimmune disorders are the most common cause of hyperthyroidism. Of these disorders, Grave's disease is the most well-known. Other conditions that can cause hyperthyroidism include:

• Multinodular goiter (enlarged thyroid)
• Thyroid inflammation (thyroiditis)
• Toxic nodules on your thyroid
• Thyroid gland dysfunction due to a medication
• Excess thyroid hormone therapy
• Other autoimmune disorders

A goiter is an enlarged thyroid. Thyroids can grow nodules on them, and if more than one grows, it is called a multinodular goiter, which produces too much thyroid hormone. These nodules can also be toxic and cause the thyroid not to function as it should.

Certain medications can also affect the production of TSH in your pituitary gland, which can cause the thyroid not to produce the correct level of hormones. Corticosteroids, dopamine, and some somatostatin drugs can inhibit the pituitary's production of TSH {Best Practice & Research Clinical Endocrinology & Metabolism: “Drugs that suppress TSH or cause central hypothyroidism.”}.

The most common and effective test for low TSH is a blood test. In addition to analyzing your TSH level, your doctor is looking for T4 (thyroxine) and T3 (triiodothyronine), which the thyroid produces. These three hormones give doctors an indication of which gland they should focus on, the thyroid or the pituitary. 

Your doctor may send you to get some imaging testing done to get a look at your glands. An ultrasound can be performed, or a thyroid scan with slightly radioactive iodine and a special camera can look for inflammation or thyroid shrinkage. 

Your thyroid may not be taking in enough iodine to produce the hormones it is supposed to — to test this, doctors use the radioactive iodine uptake test, which involves a dose of slightly radioactive iodine and a special probe to check for uptake levels. 

An antibody test can also be used to check for an autoimmune disorder, such as Grave’s disease, causing hyperthyroidism.

In many cases, doctors use radioactive iodine as a treatment for hyperthyroidism. This treatment destroys the thyroid gland over time, creating a life-long need for thyroid hormone medication. Antithyroid medication such as methimazole decreases the amount of thyroid hormone produced but is not a long-term solution. 

Doctors also sometimes see the need to remove all or part of the thyroid gland, depending on the condition's severity. Doctors discuss your treatment options with you and decide which is right based on your preferences or needs.

TSH is a glycoprotein that controls the production of two main thyroid hormones: thyroxine (T4) and triiodothyronine (T3). TSH is produced by the pituitary gland, which is part of the brain and plays an important role in performing major vital body functions.

T3 and T4 exert their actions by binding to their receptors located on the cells in the thyroid gland, which is a small butterfly-shaped gland located in front of the trachea. Upon stimulation by TSH, the thyroid gland secretes hormones in circulation, and they perform various actions such as regulating the body weight, temperature, and muscle strength.

The main functions of T3 and T4 are controlling and maintaining metabolic rate and heart health, muscle control, brain development, gut movement, and bone health. Moreover, the brain constantly monitors the levels of thyroid hormones in the blood, and if the amount is too low, it releases TSH to increase the level of thyroid hormones in the blood. When T3 and T4 levels are high, the pituitary gland produces less TSH.

The major drive for TSH production is when the body is low on T3 and/or T4:

• High TSH levels indicate low levels of thyroid hormone activity. Inadequate thyroid hormone production leads to hypothyroidism.
• Low TSH levels indicate high levels of thyroid hormone activity, most likely due to too much thyroid hormone production or hyperthyroidism, which is suppressing TSH.

High levels of TSH indicate that thyroid functions are abnormal and thyroid hormones are not being produced enough, a condition called hypothyroidism. This condition slows metabolism and can lead to following signs and symptoms:

• Weight gain
• Fatigue
• Low energy levels
• Mental fog
• Frequently being cold
• Hair loss
• Irregular menstrual cycles and fertility problems
• Muscle pain and cramps
• Slow pulse
• Low blood pressure

Thyroid hormones affect almost every cell of the body. T3 and T4 function together to regulate a series of vital functions, which include:

• Metabolic rate
• Weight loss or gain
• Heart rate
• Body temperature
• Digestion
• Muscle control
• Respiratory rate
• Bone health
• Brain development
• Cellular production of energy
• Menstrual cycle
• Growth and development in children

Conditions that can lead to abnormal levels of thyroid-stimulating hormone include:

• Post-pregnancy hypothyroidism
• Pernicious anemia
• Type I diabetes mellitus
• Primary adrenal insufficiency (Addison's disease)
• Turner syndrome
• Autoimmune disease that affects the thyroid gland
• Some cases of lupus
• Rheumatoid arthritis
• Sjogren’s syndrome
• Aging
• Radiation treatments for cancer
• Thyroidectomy (removal or separation of all or part of the thyroid gland)
• Non-functioning thyroid gland at birth
• Iodine deficiency
• Taking medications high in iodine