How Hypothyroidism Is Diagnosed

hypothyroidism diagnosis
© Verywell, 2018

If you have signs or symptoms of an underactive thyroid gland (called hypothyroidism), it's important to see your doctor for a complete evaluation. In order to check for a thyroid problem, your doctor will ask you questions about your personal and family medical history, perform a physical examination, and run blood tests (most notably, a thyroid-stimulating hormone, or TSH test).

If your doctor diagnoses you with hypothyroidism, he will also want to know the cause of your thyroid dysfunction, as this will dictate your treatment plan. To unveil the "why" behind your hypothyroid diagnosis, you may need to undergo further testing, like an antibody blood test.

History and Examination

When you see a doctor for the first time with signs or symptoms suspicious for hypothyroidism, you can expect to undergo a complete medical history and physical examination. 

After reviewing any new symptoms that signal your body's metabolism may be slowing down (for example, drier skin, tiring more easily, cold intolerance, or constipation), your doctor will ask specific questions about your medical history, such as:

  • Do you have another autoimmune disease, such as rheumatoid arthritis or type 1 diabetes?
  • Do you have any family members who have hypothyroidism?
  • Have you ever had thyroid surgery?
  • Are you taking any medications that cause hypothyroidism like amiodarone or lithium?
  • Are you taking any iodine-containing supplements?
  • Have you ever had radiation to your neck to treat lymphoma or head and neck cancer?

In addition to taking a medical history, your doctor will examine your thyroid for enlargement (called a goiter) and lumps (nodules). Your doctor will also check for signs of hypothyroidism like a low blood pressure, low pulse, dry skin, swelling, and sluggish reflexes.

Labs and Tests

The diagnosis of hypothyroidism relies heavily on blood tests.

Thyroid-stimulating Hormone (TSH)

The TSH test is the primary test used for the diagnosis and management of hypothyroidism. But different labs often have slightly different values for what is known as the "TSH reference range." 

At many labs, the TSH reference range runs from 0.5 to 4.5. A TSH value of less than 0.5 is considered hyperthyroid, while a TSH value of more than 4.5 is considered potentially hypothyroid.

Different labs might use a lower limit of anywhere from 0.35 to 0.6, and an upper threshold of anywhere from 4.0 to 6.0. In any case, it is important for you to be aware of the reference range at the lab where your blood is sent, so you know the standards by which you are being diagnosed.

If the initial TSH blood test is elevated, it's often repeated, and a free thyroxine T4 test is also drawn.

Free Thyroxine (T4)

If the TSH is high and the free T4 is low, a diagnosis of primary hypothyroidism is made. 

If the TSH is high, but the free T4 is normal, a diagnosis of subclinical hypothyroidism is made. Treatment of subclinical hypothyroidism depends on a number of factors.

For example, your doctor may treat your subclinical hypothyroidism if you have symptoms like fatigue, constipation, or depression, or you have another autoimmune disease, for example, celiac disease.

Age will also play a role in your doctor's decision. Typically, there is a higher threshold for initiating thyroid hormone replacement medication in older adults; this is because their baseline TSH is at the upper limits of normal.

The presence of TPO antibodies (see below) also plays a role in your doctor's decision. If you have subclinical hypothyroidism and positive TPO antibodies, your doctor will likely initiate thyroid hormone treatment to prevent the progression of subclinical hypothyroidism into overt hypothyroidism.

The rare diagnosis of central or secondary hypothyroidism is a bit trickier. Central hypothyroidism suggests a pituitary gland or hypothalamus problem. These brain structures control the thyroid gland and may be damaged from tumors, infections, radiation, and infiltrative diseases like sarcoidosis, among other causes. 

In central hypothyroidism, the TSH is low or normal and the free T4 is generally low-normal or low. 

TPO Antibodies

Positive thyroid peroxidase (TPO) antibodies suggest a diagnosis of Hashimoto's thyroiditis, which is the most common cause of hypothyroidism in the United States. These antibodies slowly attack the thyroid gland, so the development of hypothyroidism tends to be a gradual process, as the thyroid becomes less and less able to produce thyroid hormone.

This means that a person can have positive TPO antibodies, but a normal thyroid function for some time; in fact, it can take years for a person's thyroid function to decline to the point of being hypothyroid. Some people even have positive TPO antibodies and never progress to being hypothyroid. 

While your doctor will not likely treat you with thyroid hormone replacement medication if your TPO antibodies are positive but your TSH is within the normal reference range, he will likely monitor your TSH over time to make sure that's still appropriate.

Imaging

While blood tests are the primary means of diagnosing hypothyroidism, your doctor may order a thyroid ultrasound if he notes (or simply wants to check for) a goiter or nodules on your physical examination. An ultrasound can help a doctor determine the size of a nodule and whether it has features suspicious for cancer. Sometimes, a needle biopsy (called a fine needle aspiration, or FNA) is performed to obtain a sample of the cells within a nodule. These cells can then be examined more closely under a microscope.

In the case of central hypothyroidism, imaging is done to examine the brain and pituitary gland. For instance, an MRI of the pituitary gland may reveal a tumor, like a pituitary adenoma.

Differential Diagnosis

The symptoms of hypothyroidism are highly variable and may be easily missed or mistaken for another medical condition.

Based on Symptoms

Depending on your unique symptoms, your doctor will evaluate you for alternative medical conditions (especially if your TSH is normal). These may include:

  • Anemia
  • A viral infection (for example, mononucleosis or Lyme disease)
  • Vitamin D deficiency
  • Fibromyalgia
  • Depression or anxiety
  • Sleep apnea
  • Liver or kidney disease
  • Another autoimmune disease (for example celiac disease or rheumatoid arthritis)

Based on Blood Test Results

While primary hypothyroidism is the most likely culprit behind an elevated TSH, there are some other diagnoses your doctor will keep in mind. For instance, thyroid blood tests that support a diagnosis of central hypothyroidism may actually be due to a nonthyroidal illness.  

Nonthyroidal lllness

People who are hospitalized with a serious illness or who have undergone a bone marrow transplantation, major surgery, or heart attack may have thyroid function blood tests consistent with central hypothyroidism (a low TSH and low T4), yet their "nonthyroidal illness" does not generally warrant treatment.

In this instance, measuring blood tests called reverse T3, a metabolite of T4, can be helpful in distinguishing between true central hypothyroidism and nonthyroidal illness. A reverse T3 is elevated in nonthyroidal illness. 

In nonthyroidal illness, thyroid function blood tests should normalize once a person recovers from their illness. Although, some people develop an elevated TSH after recovery. In these people, repeating a TSH in four to six weeks usually reveals a normal TSH.

Untreated Adrenal Insufficiency

Hypothyroidism and adrenal insufficiency may coexist, as they do in a rare condition called autoimmune polyglandular syndrome. This syndrome results from autoimmune processes involving multiple glands, especially the thyroid gland (causing hypothyroidism) and adrenal glands (causing adrenal insufficiency). 

One of the biggest dangers associated with this syndrome is treating the hypothyroidism (giving thyroid hormone replacement) before treating the hypoadrenalism (which requires corticosteroid treatment), as this can result in a life-threatening adrenal crisis. Unfortunately, with this syndrome, the hypoadrenalism may be missed because of an elevated TSH and vague symptoms that overlap with those seen in hypothyroidism.

TSH-producing Pituitary Adenoma

If the TSH is elevated, it's essential that a free T4 is also checked. In primary hypothyroidism, the free T4 should be low, but if a person has a TSH-secreting pituitary tumor, the free T4 will be elevated.

Next Steps

Many people are diagnosed with hypothyroidism by their family doctor or internist. However, primary care physicians have varying experience in managing thyroid disease. 

Your first task is to learn whether or not your primary care doctor feels comfortable treating you, or if you should consult with an endocrinologist (a doctor who specializes in treating hormone disorders).

In the end, you may see an endocrinologist once, and then have your primary care doctor manage your thyroid disease moving forward. Alternatively, your endocrinologist may do all of your thyroid care year after year if this is the case.

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