TSH: What It Is, Its Test, High and Low Levels

The TSH test is a diagnostic tool that shows how well the thyroid glands are functioning. Detecting whether the thyroid glands are underactive or overactive is crucial for determining the appropriate treatment for the patient. This test can be performed for individuals already diagnosed with thyroid disease for monitoring purposes or as a screening tool to detect thyroid disorders in otherwise healthy individuals.

What Is TSH?

TSH (Thyroid Stimulating Hormone) is a hormone that stimulates the thyroid glands. TSH is released into the bloodstream from a part of the brain called the pituitary gland. It is essential for the proper functioning of the thyroid that TSH levels remain within normal limits.

The TSH test measures the level of this hormone in the blood. Based on the results, the doctor can determine whether the patient's TSH level is normal or whether it is low or high.

What Does TSH Do?

As its name suggests, TSH is a stimulating hormone for the thyroid.

The thyroid is a small, butterfly-shaped gland located in the neck. The hormones secreted by the thyroid have an impact on many functions in the body, including body temperature, muscle strength, metabolism, and mood. Therefore, for a person to be healthy, the thyroid hormones need to be secreted in adequate amounts. Abnormal secretion of thyroid hormones, whether too much or too little, can have negative effects on health.

TSH stimulates the thyroid glands to increase hormone production and secretion. There are certain feedback mechanisms that help maintain the balance between TSH and thyroid hormones. When thyroid hormone levels are low, more TSH is secreted to stimulate the production of thyroid hormones. Conversely, when thyroid hormone levels are high, the pituitary reduces the secretion of TSH. Therefore, when thyroid hormone levels are low, TSH levels are high, and when thyroid hormone levels are high, TSH levels are low.

What Is Low TSH?

The TSH test measures the levels of TSH in the blood, which provides information about thyroid function. Although the reference range for adults can vary depending on the method of measurement and the laboratory, TSH levels below 0.4 mU/L are generally considered low.

Low TSH means that the thyroid gland is more active than normal and is producing more hormones. This condition, called hyperthyroidism, leads to an accelerated metabolism. Conditions that can cause hyperthyroidism include:

• Graves' Disease (Toxic Goiter): Graves' disease is a common disorder where the thyroid gland becomes enlarged and produces an excess of hormones. It occurs due to a dysfunction in the immune system that leads to the production of thyroid-stimulating hormone receptor antibodies (TRAb). These antibodies act like TSH, stimulating the thyroid gland and causing it to work excessively.
• Thyroiditis: Thyroiditis is the inflammation of the thyroid gland. Over time, thyroiditis can lead to underactivity of the thyroid. However, in the initial stages, the inflammation causes the thyroid to release excess hormones into the bloodstream, leading to hyperthyroidism.
• Excessive Iodine: Iodine is an essential mineral for the production of thyroid hormones. An excess of iodine in the body can lead to an overactive thyroid. This usually occurs due to prolonged use of iodine-rich medications such as some cough syrups and antiarrhythmic drugs like amiodarone.
• Thyroid Nodules: Nodules are lump-like formations that appear on the thyroid gland. While most nodules are benign, some may begin producing hormones independently of the thyroid gland, leading to hyperthyroidism.

These conditions can lead to the development of hyperthyroidism. Symptoms of hyperthyroidism include:

• Weight loss despite normal or increased food intake
• Increased heart rate
• Irregular heartbeats, arrhythmia
• Palpitations
• Increased appetite
• Irritability, anxiety, restlessness
• Tremors, particularly in the hands
• Sweating
• Irregular menstrual cycles
• Increased sensitivity to heat
• Increased bowel movements, more frequent trips to the bathroom, diarrhea
• Enlargement of the thyroid gland (goiter)
• Fatigue
• Muscle weakness
• Difficulty falling asleep
• Thinning skin
• Weakened, brittle hair, hair loss

What Is High TSH?

TSH levels above 4-5 mU/L in a blood sample are generally considered high in adults. The upper limit for children is approximately the same, while for pregnant women, values above 3.5-4 mU/L are considered high.

High TSH indicates low levels of thyroid hormones. TSH rises in an attempt to increase thyroid hormone levels. A healthy thyroid responds to this elevation by producing more hormones, but when there is a problem with the thyroid gland, there may not be an increase in hormone production. Low thyroid hormone levels lead to a condition known as hypothyroidism. Conditions that can lead to hypothyroidism include:

• Hashimoto's Disease: This condition occurs when the immune system mistakenly attacks the thyroid gland. Damage caused by this condition can impair the thyroid's ability to function properly and reduce hormone production.
• Thyroiditis: Thyroiditis, the inflammation of the thyroid gland, can interfere with thyroid function in the long run, leading to hypothyroidism.
• Postpartum Thyroiditis: Some women may experience a temporary decrease in thyroid hormones after childbirth, a condition known as postpartum thyroiditis.
• Iodine Deficiency: Since iodine is necessary for the production of thyroid hormones, a deficiency in iodine can prevent adequate hormone production. In the past, iodine deficiency was common in our country, but it has become less frequent due to the addition of iodine to table salt.

Hypothyroidism has the opposite effects on the body compared to hyperthyroidism. Symptoms of hypothyroidism include:

• Weight gain
• Slowed heart rate
• Depression
• Memory problems, forgetfulness
• Irregular menstrual cycles, heavier periods
• Increased sensitivity to cold
• Constipation
• Enlargement of the thyroid gland (goiter)
• Fatigue
• Muscle pain, tenderness, and stiffness
• Joint pain, stiffness, swelling
• Dry skin
• Thinning hair
• Puffiness in the face
• Hoarseness, voice changes
• Increased blood cholesterol levels

Things to Know About the TSH Test

The TSH test is performed by analyzing a blood sample taken from a vein in a laboratory. There is no need for any special preparation, such as fasting for 8-12 hours, before the TSH test. However, if the doctor wishes to check fasting blood sugar along with TSH, the patient should fast for 8-12 hours before giving the blood sample.

In evaluating thyroid function, the TSH test is performed along with measurements of thyroid hormones T3 and T4. These values are assessed together to determine whether there is a thyroid-related issue.

Certain factors can affect the results of the TSH test. Some of these factors include:

• Timing of the Test: The TSH level measured after prolonged fasting may be slightly higher compared to a test taken when the person is in a fed state.
• Health Condition: Acute or chronic illnesses can cause stress on the endocrine system, including the thyroid gland, and may lead to deviations in test results.
• Medications: Certain medications used for arrhythmias and cancer treatments, as well as Non-Steroidal Anti-Inflammatory Drugs (NSAIDs), can affect TSH test results.

Thyroid hormones have a significant impact on the body's metabolism. Therefore, any changes in the levels of these hormones can affect the entire body. Individuals with thyroid disease should make sure to attend follow-up appointments with their doctor and undergo necessary TSH and other required tests. Healthy individuals should also periodically check their thyroid function. Both hyperthyroidism and hypothyroidism can lead to serious health issues if left untreated. Therefore, individuals with thyroid-related conditions should not neglect their treatments.