TSH Tests, and 3rd and 4th Generation Testing

In this article

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The thyroid hormones T4 (thyroxine) and T3 (triiodothyronine) help keep your cells and body functioning correctly. Thyroid hormone is responsible for your metabolism, growth and development, and even the strength of your hair and nails.

The best way to determine how well your thyroid functions is by measuring key thyroid biomarkers: TSH (thyroid stimulating hormone), T4, and T3. Abnormal thyroid biomarkers may signal a potential thyroid disorder. Your healthcare provider may also evaluate your thyroid antibody levels to determine if you have an autoimmune thyroid condition. 

This Q&A style article will look at the TSH test and the difference between this test and the TSH 3rd and 4th generation tests. 

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The TSH test measures the level of thyroid stimulating hormone (TSH) in your bloodstream.

TSH is a hormone secreted by the pituitary gland to help balance thyroid hormone production by communicating with the thyroid gland. The pituitary secretes TSH when your T4 level is low. The TSH signals your thyroid to start producing more T4. As T4 increases, TSH secretion from your pituitary gland then decreases. Finally, once T4 reaches a certain level, your pituitary gland will stop releasing TSH.  As your cells convert T4 to T3 (the active form of thyroid hormone), T4 levels drop, stimulating the release of TSH from your pituitary gland, and the cycle continues. This continuous feedback loop happens constantly.

Elevated TSH, then, is associated with hypothyroidism, and low TSH levels are associated with hyperthyroidism. Many physicians consider the TSH test the most sensitive marker for screening for thyroid disorders. So, it’s no surprise that most healthcare providers order it as one of the initial tests to assess thyroid function. And for those living with a thyroid disorder, checking TSH levels allows your healthcare provider to determine how well your treatment plan is working.

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TSH tests are named by “generation” and are classified based on the lowest detection limit of serum or plasma TSH levels from the first to fourth generations.

• The lowest detection limits of the 1st generation TSH test, which is currently not in use, was 1–2 µIU/mL
• The 2nd generation TSH kits (fast TSH tests) have a detection limit of 0.1–0.2 µIU/mL
• The 3rd generation TSH test detection limit is 0.01–0.02 µIU/mL
• The 4th generation TSH test detection limit is 0.001–0.002 µIU/mL

The main difference between a regular TSH test and a 3rd generation or 4th generation TSH test is the level of sensitivity and accuracy in detecting TSH levels in the blood. TSH 3rd and 4th generation assays are more advanced and sensitive versions of the standard TSH test and accurately measure very low levels of TSH. These tests are considered more precise and reliable in detecting subtle changes in TSH levels, allowing for better management of thyroid conditions.

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The American Thyroid Association Guidelines for treating hypothyroidism consider a TSH value between 0.4–4.0 mIU/L to be within the reference range, also called the normal range. However, this range may vary slightly from lab to lab. The range also differs somewhat based on your age or pregnancy status.

These guidelines also suggest a more lenient TSH range for older adults, meaning a higher TSH may be appropriate for this population. Some studies suggest a TSH above 7 mIU/L is acceptable for individuals over 80 because TSH levels naturally increase with age. Higher TSH values in older adults are associated with improved well-being and lower death rates. Despite the positive evidence suggesting a higher TSH in older individuals may be better, there is no set range, and this topic is very controversial.

Regarding children, acceptable TSH ranges vary greatly depending on the child’s age. Higher TSH values are more common in newborns and those under one-year-old. As children age, the acceptable TSH range becomes closer to an adult’s.

For pregnant women with hypothyroidism, the TSH range changes based on the trimester. An abnormally high TSH during pregnancy, as seen in those with an untreated or undertreated thyroid disorder, is associated with maternal and fetal complications.

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An abnormal TSH test may indicate a thyroid disorder. But this shouldn’t be the only factor considered when diagnosing a thyroid disorder.

High TSH level

A high TSH level generally means your thyroid gland is underactive or not producing enough thyroid hormone. Because of this, you may have a medical condition called hypothyroidism resulting in symptoms of:

• Cold intolerance
• Constipation
• Dry, brittle hair or nails
• Joint and muscle pain
• Mental health changes
• Tiredness
• Reproductive changes such as infertility or heavy periods
• Weight gain

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Low TSH level

In contrast, a low TSH level signifies your thyroid gland is overactive. High thyroid hormone levels may lead to you being diagnosed with hyperthyroidism. Symptoms of hyperthyroidism include:

• Diarrhea
• Fatigue
• Heat intolerance
• Muscles weakness
• Nervousness
• Racing heart
• Weight loss

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While you may be excited to see a value within the normal range, this news can also create frustrations if you have thyroid symptoms! Symptoms of thyroid disorders can be non-specific and may be due to other medical conditions, making it challenging to find the underlying cause. Potential causes of thyroid disorder symptoms with a normal TSH include:

• Subclinical hypothyroidism or hyperthyroidism is when your TSH is mildly abnormal, but your thyroid hormone levels are normal. In comparison, those with overt hypothyroidism or hyperthyroidism have abnormal TSH and thyroid hormone levels.
• Vitamin and nutrient deficiencies such as iron, folate, vitamin D, and vitamin B-12 may cause symptoms similar to hypothyroidism. Correcting these underlying deficiencies may improve your symptoms.
• Stress, especially chronic, can alter how your body functions, leading to unwanted symptoms such as fatigue, headaches, and mood swings.
• Poor thyroid hormone conversion, whether related to your diet, other medical conditions such as diabetes, or environmental factors, can cause symptoms similar to thyroid dysfunction.

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When you’re having a standard thyroid blood test – whether at your doctor’s office or a lab – there’s one thing to remember if you are supplementing with biotin. Biotin, also known as vitamin B7, may interfere with accurately measuring your thyroid function, according to a small 2021 study. A daily dose of 10 mg or more of biotin may cause your reported TSH value to be lower than it really is. Biotin interference may result in an incorrect diagnosis or changes to your thyroid hormone replacement medication. (Note: Biotin intake does not affect the TSH results on Paloma’s complete home thyroid test kit.) 

If you take biotin supplements, ask your healthcare provider how long you should stop taking them before having your blood drawn. The 2021 study referenced earlier recommends holding your biotin supplements for at least 24 hours. But specific lab tests may require you to stop taking them at least 72 hours before your lab visit for the most accurate results.

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As mentioned, testing your TSH requires a blood sample. Typically, this is done by getting a lab order from your healthcare provider and visiting a local lab during their business hours to have your blood drawn or having the blood drawn at your doctor’s office.

Remember, there’s also a simpler way to test your TSH and other thyroid biomarkers. Paloma’s at-home testing kit is done from the comfort of your home when it is most convenient for you. Testing your TSH, Free T4, Free T3, and thyroid peroxidase (TPO) antibodies involves a painless and easy finger-prick blood sample. And, as noted, if you take biotin supplements, you don’t need to worry about stopping them!

If your lab values come back abnormal, schedule an appointment with one of Paloma’s thyroid providers. Here at Paloma, we take an individualized approach to managing hypothyroidism.