Thyroid-stimulating hormone (TSH) is a hormone that prompts your thyroid gland to produce the thyroid hormones triiodothyronine (T3) and thyroxine (T4). The hypothalamus, located in the brain, constantly monitors your blood for levels of thyroid hormones, and if it detects too little, it triggers the pituitary gland, also located in the brain, to release TSH.

This system of communication keeps your thyroid hormone levels within the optimal range. If you have enough or too much T3 or T4, the pituitary gland slows down the production of TSH, which signals your thyroid to produce less T3 and T4.

Thyroid Hormone Functions

The thyroid gland is front and center at the base of your neck, just below your larynx (voice box) and above your sternum. It’s shaped like a butterfly, with two lobes that are joined in the center by a narrow strip of tissue. The hormones made in your thyroid gland are essential to your metabolism, growth, and development.

T3 and T4 work together, along with other hormones in your body to regulate many vital functions, including:

  • Metabolic rateWeight loss or gainHeart rateBody temperatureDigestionMuscle controlRespiratory rateBone healthBrain developmentCellular production of energyMenstrual cycleGrowth and development in children

Thyroid hormones literally have an impact on every cell in your body, so when their levels are not within normal ranges, that can have a major impact on your health.

Causes of Abnormal TSH Levels

Several different things can cause your TSH levels to be abnormal.

Some conditions that can affect your thyroid gland include:

  • Primary adrenal insufficiency (Addison’s disease)
  • Autoimmune disease that affects the thyroid gland (Hashimoto’s disease, Graves’ disease)
  • Radiation treatments for cancer of or near the thyroid gland
  • Thyroidectomy (removal of all or part of the thyroid gland)
  • Non-functioning thyroid gland at birth
  • Iodine deficiency
  • Taking medication that’s high in iodine

Other factors associated with TSH abnormalities include:

  • Aging
  • Pernicious anemia
  • Diabetes type 1
  • Turner syndrome

Associated Conditions

High TSH levels are associated with low levels of thyroid hormones, and low TSH levels are associated with high levels of thyroid hormones.

Iodine is essential for the proper production and function of the thyroid hormones. Your body cannot produce iodine, though, so you have to get it through your diet or nutritional supplements. Some natural sources of iodine are:

  • Fish and other seafood
  • Dairy products
  • Grains
  • Iodized salt

When levels are slightly abnormal and without symptoms, it’s classified as a subclinical thyroid disorder. Some people diagnosed with a subclinical thyroid disorder eventually go on to develop full-blown thyroid disease.

A condition called thyroiditis is a type of inflammation of the thyroid gland that interferes with hormone production. It can be associated with either high levels or low levels of TSH.

Hyperthyroidism

Also called overactive thyroid, this condition accelerates your metabolism.

  • Hyperthyroidism and Graves’ disease are associated with low levels of TSH and high thyroid function. Hypothyroidism and Hashimoto’s disease are associated with high levels of TSH and low thyroid function.

Symptoms include:

  • Unintended weight lossIncreased sweatingRapid heart rateIrregular heart rhythmsHigh blood pressureShortness of breathMalabsorption of nutrientsFrequent urinationInfrequent menstrual periods or no periods at allErectile dysfunctionTremorAnxiety

Graves’ Disease

In this autoimmune condition, the immune system’s attack on the thyroid gland causes it to malfunction and produce excess hormones. The symptoms are generally the same as those of hyperthyroidism.

In addition, Graves’ is often associated with:

  • Goiter (enlarged thyroid gland)
  • Graves’ ophthalmopathy
  • Skin lesions

Hypothyroidism

Also called underactive thyroid, this condition slows the metabolism and can lead to:

Graves’ ophthalmopathy develops due to the autoimmune process. It can give your eyes a bulging appearance because it causes increased pressure around the eyes. It can also cause eye sensitivity and decreased vision.

  • Weight gainFatigueLow energy levelsMental fogFrequently being coldHair lossIrregular menstrual cycles and fertility problemsMuscle pain and crampsSlow pulseLow blood pressure

Hashimoto’s Disease

Sometimes called Hashimoto’s thyroiditis, this is an autoimmune disease in which the immune system attacks healthy thyroid tissues. Symptoms are similar to those of hypothyroidism but may be more severe.

Some women experience a drop in thyroid function after giving birth, which is called postpartum thyroiditis. This is typically a temporary condition, with the function returning to normal over time.

TSH Testing Interpretations

TSH is an important marker for determining whether someone has thyroid disease.

Sometimes, a healthcare provider will order tests for TSH, free T3, and free T4 together, possibly with other tests to get a more complete picture of how well your thyroid gland is functioning.

A Word From Verywell

If you have symptoms that may be suggestive of a thyroid disorder, it’s important for you to get an appointment with your healthcare provider. These disorders are common and treatable, so there’s no reason for you to continue dealing with unpleasant—and potentially dangerous—symptoms.

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By Adrienne Dellwo

Adrienne Dellwo is an experienced journalist who was diagnosed with fibromyalgia and has written extensively on the topic.