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Hypothyroidism can be diagnosed in a few ways, but the most reliable way to determine whether a person has an underactive thyroid is through blood tests.

Because there are no specific clinical manifestations of an underactive thyroid, the easiest way to diagnose the disease is by conducting laboratory testing. While trying to identify the causes for your underactive thyroid, your doctor will probably consider the following factors:

  • Symptoms
  • Your family's medical history
  • Risk factors
  • Physical examination
  • Laboratory blood tests to determine the level of thyroid-stimulating hormone (TSH)

Tests tend to show different types of hypothyroidism, depending on the level of hormone deficiency:

  • Primary hypothyroidism
  • Secondary hypothyroidism
  • Subclinical hypothyroidism
The primary type of hypothyroidism is characterized by high levels of TSH and low levels of serum free thyroxine (T4). Secondary hypothyroidism is identified by low levels of T4 and low levels of thyroid-stimulating hormone. Subclinical underactive thyroid is characterized by normal T4 levels, and increased levels of thyroid-stimulating hormone.

Triiodothyronine (T3) hormone mostly stays normal, sometimes even unchanged, even when T4 and TSH levels drastically change. This means that measuring T3 levels is mostly of no use. Doctors usually ask for three tests when trying to diagnose an underactive thyroid:

  • A TSH test
  • A thyroxine test
  • A test that looks for the presence of TPO antibodies in the blood

Testing your levels of thyroid-stimulating hormone

Simply put, underactive thyroid is a disease caused by low levels of the thyroid-stimulating hormones triiodothyronine and thyroxine. A serum TSH test is considered the best way to diagnose hypothyroidism because T3 and T4 hormones are released into the bloodstream through TSH. A serum TSH test measures the level of TSH hormone and if the test shows it's raised, the test is repeated after two to eight weeks to confirm the result.

These tests are reliable nowadays and they can detect even the slightest abnormalities in your thyroid function.

Normal ranges of TSH vary, and they can even differ in the morning and the evening. These ranges are expressed in milliunits per liter (mU/L) and can also vary slightly between labs. Thyroid-stimulating hormone levels between 0.5 and 4.0 mU/L are considered normal. If it's higher than 5.0 mU/L, a person should probably undergo further testing such as a free thyroxine test (FT4) or free T4 index (FTI).

If a person has thyroid-stimulating hormone anywhere between 4.1 and 5.0 mU/U t's called a “gray zone” a they will probably get hypothyroidism one day and should get tested regularly.

This is how the American Thyroid Association classifies different levels of TSH:

  • up to 0.4 mU/L: normal level
  • 2.5 mU/L – a person is at risk of having underactive thyroid
  • 4.0 mU/L – mild or subclinical underactive thyroid
  • 10. mU/L – serious hypothyroidism

When it comes to levels of TSH, they can vary among people and depend mostly on age and gender, but also a few other factors. Thyroid-stimulating hormone differs among women in different age. For example, a woman in her thirties has normal TSH level between 0.4 and 2.34 mU/L, and elderly women can have anywhere between 0.46 and 4.68mU/L and still be considered healthy.

It's crucial to have your TSH levels monitored constantly while pregnant because hypothyroidism can increase chances of miscarriage.

Levels of thyroid-stimulating hormone differ between men and women and men's often range between 0.5mU/L to 4.15 mU/L for younger men, and between 0.4 to 5.9 mU/L for the elderly population. Thyroid-stimulating hormone levels vary between different age groups when it comes to children as well. Normal ranges of thyroid-stimulating hormone for children are:

  • 0.7 to 27 mU/L for premature born babies
  • 1.0 to 29 mU/L within the first five days of a baby's life
  • 1.7 to 9.1 mU/L from two days to two weeks old
  • .7 to 64 mU/L from 2 week of age until 18th birthday

When a baby is born, her TSH levels are usually higher in the first few months of her life, slowly decrease towards adulthood, and rise again in older age. Anything over the aforementioned numbers may suggest an underactive thyroid in children.

Free thyroxine (T4) test

Thyroxine is of great importance because in cases when thyroid-stimulating hormone test shows no sign of hypothyroidism, it doesn't rule out underactive thyroid as a diagnosis. If TSH levels are increased and it doesn't suggest hypothyroidism, your physician can suggest a free T4 test, where they take part of your T4 hormone available in the tissues. It can happen that the person has high levels of thyroid-stimulating hormone and low thyroxine levels, which can mean that they have what's known as primary hypothyroidism.

This happens because the thyroid gland is not making enough TSH, so the pituitary gland produces more and more thyroid-stimulating hormone, just to trigger the gland to produce more hormone. In cases when the thyroid gland is not working as it should, it won't recognize these signals that the pituitary gland sends and it will cause high levels of thyroid-stimulating hormone and decreased levels of thyroxine.

Most experts consider having between 5.0 and 13.5 micrograms per deciliter the normal range for T4. Levels of the hormone that are lower than recommended are considered a possible sign that a patient may suffer from an underactive thyroid, even in cases when previous tests showed no hypothyroidism.

Anti-TPO blood test

An anti-thyroid peroxidase test (shortened to anti-TPO) is a third type of testing done to determine hypothyroidism. This test discovers anti-thyroid microsomal antibodies produced by our own immune system that attack thyroid cells. If this test confirms the presence of antibodies, a person might be at risk of developing hypothyroidism. Hashimoto's disease is one of the conditions caused by these antibodies. Your physician probably won't conduct this test, but they can refer you to an endocrinologist.

This test is not a common part of a hypothyroidism diagnosis, but it's definitely useful. A positive test to these antibodies is greatly associated with having an underactive thyroid.

An anti-TPO test is recommended for people with mild cases of underactive thyroid (subclinical hypothyroidism), where the antibodies commonly mean increased risks for the overt type of hypothyroidism. If a person suffers from some autoimmune disorder such as Addison disease or diabetes type 1, or a chromosomal disease such as Turner or Down syndrome, these antibodies represent a predisposition to hypothyroidism.

Here are the normal antibody ranges just for reference:

  • Thyroglobulin should be less than 20 IU/mL
  • Thyroid peroxidase should be less than 35 international units per milliliter (IU/mL)
  • Thyroid-stimulating immunoglobulin antibodies should be less than 14 percent of main activity

Other factors that can affect your thyroid health

When it comes to evaluating laboratory test results for hypothyroidism, a few factors must be considered because you can get similar results by a few other factors such as:

  • Acute illness in hospitalized patients may cause low serum TSH and low FT4.
  • During recovery from an illness (it doesn't have to be even thyroid-related) thyroid-stimulating hormone may increase significantly.
  • Pregnancy may cause serum TSH drop within the first three months, but the hormone goes back to normal after the first trimester.
  • After delivery, TSH and FT4 may increase and lead to a condition known as postpartum thyroiditis. This condition eventually goes away on its own.
  • Anorexia may decrease the levels of TSH and FT4 due to pituitary dysfunction.
  • Aging can lead to mild hypothyroidism. TSH that is over 3.0 mLU/L often occurs in otherwise healthy elderly people.

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