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How Doctors Diagnose Hypothyroidism

Diagnosis of underactive thyroid function not based on symptoms alone

Hypothyroidism is evaluated and diagnosed by a physician; usually an endocrinologist or your primary care doctor. Symptoms, signs, and several factors are taken into consideration when hypothyroidism is diagnosed—all of which help identify the cause and severity of the disease. A diagnosis is reached after a thorough review of the patient’s symptoms, medical and family history, risk factors, physical examination, and most effectively, a blood test. There are several types of blood tests—the most definitive one is called the TSH test (thyroid-stimulating hormone). However, in some cases, physicians may refer to the free thyroxine or T4, free T4 index, or total T4 to aid in the diagnosis.

Why Hypothyroidism is not Diagnosed on Symptoms Alone
Many of the symptoms of hypothyroidism are fairly common complaints found in people with a normally functioning thyroid gland, so it can be hard to decipher if the symptoms are related to the thyroid. One of the best ways to figure out if your symptoms could be related to a thyroid condition is to consider how long you have been experiencing them.  For example, have you always felt cold when others were warm? Did you just start to notice decreased energy? If you are starting to notice new signs and symptoms, it could be related to a thyroid issue. However, only a physician (eg, endocrinologist) can diagnose a thyroid problem.

Medical and Family History
It’s important to give your physician as many details as possible about your personal medical history, as well as family history (eg, mother had hypothyroidism). Be sure to discuss:

  • Your general state of health—particularly any changes you have noticed in your general overall health.
  • Your family’s health history—especially if a close relative has been diagnosed with hypothyroidism (or any other thyroid-related issues).
  • Whether you’ve ever had thyroid surgery, or radiation to your neck to treat cancer.
  • Any medicines you may be taking that could cause hypothyroidism (eg, amiodarone, lithium, interferon alpha, interlukin-2, or prior chemotherapy).

Physical Examination—Looking for Signs of Hypothyroidism
Your physician will perform a thorough examination and look for physical signs of hypothyroidism, including:

  • Evidence of dry skin
  • Swelling around the eyes and legs
  • Slower reflexes
  • Slower heart rate

Blood Tests

Hypothyroidism can be detected by different blood tests.

TSH Test
A thyroid-stimulating hormone or TSH is a blood test that measures the amount of T4 (thyroxine) that the thyroid is being signaled to make. If you have an abnormally high level of TSH, it could mean you have hypothyroidism.

T4 (thyroxine) Test
The thyroid gland produces T4 (thyroxine). The free T4 and the free T4 index are blood tests that, in combination with a TSH test, can let your physician know how your thyroid is functioning. 

The pituitary gland tells the thyroid how much thyroxine to produce through signaling by TSH. There are cells in the pituitary gland that decide what your body’s “set point” is. Your set point is the normal range of TSH your body needs, as determined by your pituitary gland.

As blood flows through the pituitary gland, these same cells detect if there are adequate T4 levels in the body. If your T4 level is adequate, the pituitary sends the appropriate amount of TSH to the thyroid to maintain T4 levels in the normal range. If your T4 level is too low, the pituitary sends more TSH out telling the thyroid to make more T4. If your T4 level is too high, the pituitary sends less TSH out, telling the thyroid to make less T4.

Normal and Abnormal TSH Ranges

  • 0.4 mU/L to 4.0 mU/L is considered the reference range (there may be slight variation depending on the laboratory), and people who have a normally functioning thyroid gland usually fall within this range.
  • If TSH measures >4.0 mU/L, a second test (T4) is performed to verify the results. TSH >4.0/mU/L with a low T4 level indicates hypothyroidism.
  • If your TSH is >4.0 mU/L and your T4 level is normal, this may prompt your physician to test your serum anti-thyroid peroxidase (anti-TPO) antibodies. When these antibodies are present, it may indicate an autoimmune thyroid disorder, which is a risk factor for developing hypothyroidism.  If you have these anti-bodies, your doctor will most likely perform and TSH test at least once per year.

An easy way to remember how the thyroid works is to think about supply and demand. As the T4 level falls, the TSH rises. As the T4 level rises, the TSH falls. However, not everyone with hypothyroidism has elevated levels of TSH. If the pituitary is not working properly, it may not send out normal TSH amounts—and if this is the case—the thyroid may be healthy. However, if the amount of TSH is off, the thyroid won’t make the right amount of T4. This is rare and is known as secondary or central hypothyroidism.

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