What Is Thyroid Stimulating Hormone?

Thyroid stimulating hormone (TSH) is a hormone that’s produced by the pituitary gland in your brain for the single purpose of sending a message to the thyroid gland. The pituitary gland constantly monitors your blood for levels of thyroid hormones, and if it detects too little, it releases TSH. That tells your thyroid gland to produce more of the thyroid hormones triiodothyronine (T3) and thyroxine (T4).

If the pituitary detects too much T3 or T4, it slows down the production of TSH, which signals to your thyroid that it should produce less T3 and T4. When both glands are functioning properly, this system of communication keeps your hormone levels within the optimal range.

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 much like a butterfly, with two lobes that are joined in the center by a narrow strip of tissue. The thyroid gland's job is to make hormones that are essential to your metabolism, growth, and development.

T3 and T4 work together to regulate an array of vital functions, including:

• 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

Thyroid hormones literally have an impact on every cell in your body, so when their levels are outside of normal parameters, it can have a major impact on your health.

Causes of Abnormal TSH Levels

Several different things can cause your TSH levels to be abnormal. Often, it’s due to a medical condition that impairs the thyroid gland. Some conditions that are known to do this include:

• Pernicious anemia
• Type 1 diabetes
• Primary adrenal insufficiency (Addison's disease)
• Turner syndrome
• Autoimmune disease that affects the thyroid gland (Hashimoto’s disease, Graves’ disease, and some cases of lupus, rheumatoid arthritis, and Sjogren’s syndrome)

Other known causes of TSH abnormalities include:

• Aging
• 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

Associated Conditions

Because it’s produced when your body is low on T3 and/or T4, high TSH test results are associated with low levels of thyroid hormone and activity. Conversely, low TSH levels are associated with high levels of thyroid hormone and activity.

When levels are abnormal but only by a small amount and no symptoms are present, 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, which features inflammation of the gland that interferes with hormone production, can be associated with either high levels or low levels of TSH.

Diseases associated with low levels of TSH/high thyroid function are hyperthyroidism and Graves' disease.

Hyperthyroidism

Also called overactive thyroid, this condition accelerates your metabolism. Symptoms include:

• Unintended weight loss
• Increased sweating
• Rapid heart rate
• Irregular heart rhythms
• High blood pressure
• Shortness of breath
• Malabsorption of nutrients
• Frequent urination
• Infrequent periods or no periods at all
• Erectile dysfunction
• Tremor
• Anxiety

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

Diseases associated with high levels of TSH/low thyroid function are hypothyroidism and Hashimoto's disease.

Hypothyroidism

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

• 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

Hashimoto's Disease

Sometimes called Hashimoto's thyroiditis, this is an autoimmune disease in which the immune system misidentifies healthy thyroid tissues as a pathogen, such as a virus or bacteria, and tries to destroy it. 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 doctor will order tests for TSH, free T3, and free T4 together, possibly with other tests as well. This presents a much more complete picture of how well your thyroid is functioning than just a TSH test.

A Word From Verywell

Having thyroid stimulating hormone in the proper amounts is crucial to your health. If you have symptoms that may be suggestive of a thyroid disorder, it's important for you to get an appointment with your doctor. These disorders are common and treatable, so there’s no reason for you to continue dealing with unpleasant—and potentially dangerous—symptoms.