Thyroid-Stimulating Hormone

Thyroid Stimulating Hormone (TSH) serves as a critical regulator of thyroid function, exerting influence over various bodily processes. This hormone, produced by the pituitary gland, plays a pivotal role in maintaining thyroid hormone levels within the body. 

Understanding TSH levels is essential for evaluating thyroid health, as deviations from the normal range can indicate underlying thyroid dysfunction.  Understanding the significance of TSH in health and wellness provides valuable insights into maintaining optimal thyroid function and overall well-being.

Definition of TSH: What is TSH?

Thyroid Stimulating Hormone (TSH) is a glycoprotein hormone produced in the brain.  It is released into circulation in response to decreasing levels of systemic thyroid hormone.

The production site of TSH is the anterior pituitary gland, a pea-sized structure in the brain. Within the anterior pituitary, specialized cells known as thyrotrophs synthesize and secrete TSH in response to signals from the hypothalamus. 

The hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates thyrotrophs to release TSH into the bloodstream. Once in circulation TSH travels to the thyroid gland, where it binds to specific receptors on thyroid follicular cells: this initiates the production and secretion of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), with T3 being the biologically active form of thyroid hormone.

This intricate feedback loop ensures that thyroid hormone levels remain within a narrow physiological range, with TSH acting as a key mediator in the regulation of thyroid function.  As such, TSH levels serve as a sensitive indicator of thyroid function and are commonly measured in clinical settings to assess thyroid health.

Clinical Implications of TSH Levels

High Levels of TSH: What Does High TSH Mean?

High levels of thyroid-stimulating hormone (TSH) often indicate hypothyroidism, a condition where the thyroid gland fails to produce sufficient thyroid hormones. 

Clinically, elevated TSH signifies that the body is attempting to stimulate the thyroid to increase hormone production. An elevation in TSH may suggest an underactive thyroid, which can lead to symptoms such as fatigue, weight gain, constipation, and sensitivity to cold.

Less commonly, elevated TSH may be caused by central hyperthyroidism.  In central hyperthyroidism, excessive TSH is produced by the pituitary gland, leading to thyroid enlargement and hyperfunction.  

Low Levels of TSH: What Does Low TSH Mean?

Low levels of thyroid-stimulating hormone (TSH) typically suggest hyperthyroidism, characterized by an overactive thyroid gland producing excessive thyroid hormones. 

Clinically, a decreased TSH level indicates that the body is trying to suppress thyroid hormone production to restore balance. This decrease in TSH may lead to symptoms like weight loss, increased heart rate, tremors, anxiety, and heat intolerance.

Less commonly, low TSH levels may also indicate central hypothyroidism.  In central hypothyroidism, the problem lies in the hypothalamus or pituitary gland, or with hypothalamic-pituitary circulation, leading to decreased TSH levels, TRH levels, or both.  [19.]

What is Subclinical Hypothyroidism?

Subclinical hypothyroidism is a condition where the thyroid-stimulating hormone (TSH) levels are elevated, indicating mild thyroid dysfunction, but the thyroid hormone levels, particularly thyroxine (T4), remain within the normal range. 

This condition often lacks noticeable symptoms but can sometimes lead to subtle manifestations such as fatigue, weight gain, and dry skin. While not as severe as overt hypothyroidism, subclinical hypothyroidism may still pose health risks, including an increased likelihood of developing overt hypothyroidism in the future, as well as an increased risk of cardiovascular disease and cognitive decline.  [9.]

Management strategies for subclinical hypothyroidism typically involve regular monitoring, diet and lifestyle modifications as listed below, and in some cases, consideration of thyroid hormone replacement therapy, especially in individuals with high TSH levels or specific risk factors.

Testing for TSH

TSH Blood Test Overview and Preparation

The TSH blood test, also known as thyroid-stimulating hormone test, measures the level of TSH in the blood.  It is an essential marker of thyroid function.  

The TSH blood test sample is typically obtained through venipuncture, drawing blood from a vein in the arm; it is not necessary to fast for this test.  However, per the American Association of Clinical Endocrinologists and the American Thyroid Association, patients taking thyroid medication should have their blood drawn prior to taking their daily dose of thyroid hormone.   [7.]

Patients taking biotin supplements should discontinue these supplements for 72 hours prior to testing, as they may interfere with the test results.  

TSH Testing Options

TSH testing is a blood test.  Traditional blood tests involve drawing blood from a vein, typically from the arm, and sending it to a laboratory for analysis. 

Some labs will offer TSH testing as a blood spot test, and this option allows individuals to take a blood sample at home and send it to a lab for monitoring.  

TSH with Reflex to FT4

TSH with reflex to FT4 testing is a comprehensive approach that not only measures TSH levels but also assesses the level of free thyroxine (FT4), one of the thyroid hormones.  This reflex testing is triggered when TSH levels fall outside the normal range, prompting further evaluation of thyroid function. 

Specifically, if TSH levels are either too low or too high, the test will do an additional assessment of free, or bioavailable, T4 levels in the body.  [2.]  Reference ranges may differ by labs, but typical levels for a TSH reflex to T4 test are 

TSH <0.450

or

TSH >4.5

This reflex test provides additional immediate assessment of excessive or decreased thyroid function, aiding in the diagnosis of hyper- or hypothyroidism.  

Interpreting TSH Levels

Normal TSH Levels

The normal TSH range typically falls between 0.450−4.500 milli-international units per liter (mIU/L), although this range can vary slightly depending on the laboratory and specific testing method used.  [1.]

Once patients begin thyroid hormone therapy for hypothyroidism, the American Thyroid Association recommends that TSH levels remain between 0.4-4.0 mIU/L per one study of over 162,000 patients, which monitored health risks associated with TSH levels over 22 years.  [18.]  

What is a High TSH Level?

Elevated TSH levels beyond the normal range of about 4.500 mIU/L often indicate hypothyroidism, a condition where the thyroid gland fails to produce sufficient thyroid hormones. This can lead to symptoms such as fatigue, weight gain, cold intolerance, and sluggishness.

What is a Dangerously High TSH Level?

A dangerously high TSH level, often indicative of hypothyroidism, typically exceeds 10 mIU/L.  This was reinforced by the study referenced above, which determined the following: [18.]

• Elevated TSH levels exceeding 10 mIU/L were associated with a heightened risk of heart disease due to blood vessel damage. 
• The risk of stroke was slightly reduced within the TSH range of 3-3.5 mIU/L and 4-10 mIU/L. 
• Overall mortality risk increased notably when TSH levels dropped below 0.1 mIU/L or rose above 10 mIU/L. 
• The incidence of broken bones, particularly among women aged 65 and older, was more prevalent at TSH levels exceeding 10 mIU/L.

What is a Low TSH Level?

TSH levels below the normal range of 0.4 mIU/L may suggest hyperthyroidism, where the thyroid gland produces excessive thyroid hormones. This can manifest in symptoms such as weight loss, rapid heart rate, heat intolerance, and anxiety.

Managing High or Low TSH

Strategies to Lower TSH 

Managing high TSH levels typically involves addressing the underlying cause, which may include hypothyroidism, thyroiditis, or in some cases, iodine deficiency. 

Treatment options vary depending on the specific condition but may include thyroid hormone replacement therapy, such as levothyroxine, to restore thyroid hormone levels and normalize TSH secretion. 

Specific Recommendations for Supporting Thyroid Health Include:

Eat a Balanced Diet: consuming a diet rich in whole grains, lean proteins, fruits, and vegetables provides essential nutrients like iodine, selenium, zinc, and vitamins B and D, which support thyroid function.

Monitor Iodine Intake: ensure adequate but not excessive iodine intake, as iodine is crucial for thyroid hormone synthesis but excessive intake can also be detrimental.  [6., 15.]

Manage Stress: practice stress-reducing techniques such as meditation, deep breathing exercises, or yoga, as stress can negatively impact thyroid function.  Additionally, hypothyroidism and elevated cortisol levels are commonly seen together, and chronic stress increases the conversion of T4 to reverse T3, the biologically inactive form of thyroid hormone.  [8., 11., 12., 20.]

Get Regular Exercise: regular physical activity supports metabolism and overall well-being, which can benefit thyroid health.  Additionally, aerobic exercise has been shown to increase available thyroid hormone levels.  [3.]

Limit Processed Foods: reduce consumption of processed foods high in refined sugars and unhealthy fats, as they can contribute to inflammation, alter thyroid hormone production, and may promote autoimmunity.  [13., 21.]

Avoid Goitrogens: limit intake of goitrogenic foods such as soy, raw cruciferous vegetables (like broccoli and cabbage), and certain fruits, as they can interfere with thyroid hormone production when consumed in large amounts.  [5.]

Get Enough Sleep: prioritize quality sleep to support hormone regulation and overall health, as inadequate sleep can disrupt thyroid function.  [14.] 

Avoid Environmental Toxins: minimize exposure to environmental toxins such as pesticides, heavy metals, fluoride in water, and endocrine-disrupting chemicals found in plastics and household products, as they can interfere with thyroid function and hormone production.  [4., 16., 17.]

Monitor Medications: speak with your prescribing doctor if you are taking any medications that may affect thyroid function, such as lithium, amiodarone, certain antiretrovirals, and others.  [10.] 

Regular Thyroid Check-ups: Schedule regular check-ups with a healthcare provider to monitor thyroid function and address any concerns promptly.