Thyroid Function Tests (Basic)

Why test for Thyroid Function Tests (Basic)

You should make an appointment with your clinican and ask for a blood test for Thyroid Function Tests if you have:

• Symptoms of an over- or under-active thyroid
• Swelling or thickening in the neck
• An irregular or fast heart rate
• High cholesterol (which causes atherosclerosis – a build-up of fat in the arteries)
• Osteoporosis (fragile or thinning bones)
• Fertility problems, abnormal menstrual cycles, recurrent miscarriage, low libido
• Family history of autoimmune disorders, e.g., type 1 diabetes, vitiligo, etc
• Feeling unwell after having a baby
• Planning pregnancy or in early pregnancy (and you have a family history or personal history of thyroid disorders, a past history of postpartum thyroiditis, or type 1 diabetes)

You should have a blood test once a year, or more frequently if your doctor advises, if:

• You have a diagnosed thyroid disorder
• You have had previous treatment for an over-active thyroid (radioactive iodine, thyroid surgery, medication)
• You have had irradiation to the head and neck after surgery for head and neck cancer
• Before you have treatment with amiodarone or lithium, then 6-12 months during treatment and 12 months after treatment
• People with Down’s syndrome, Turner syndrome, Addison’s disease or other autoimmune diseases should also be tested regularly.

How can blood tests be used to manage thyroid disorders?

The aims of treatment are to make you feel better and to ensure that you come to no long-term harm from your thyroid hormone replacement. The blood test for TSH, which is the most sensitive marker of your thyroid status, is used as a biochemical marker to ensure that your thyroid hormone replacement is adequate.
The recommended target range for TSH for patients on thyroid hormone replacement should preferably be within the reference range. Over-replacement may cause long-term harm to the cardiovascular system and the bones. The exception is thyroid cancer where the aim in selected patients is to keep the TSH level suppressed just below the reference range (usually to 0.1-0.5 mU/L).

Occasionally patients only feel well if the TSH is below normal or suppressed. This is usually not harmful as long as the FT3 is clearly normal. There are also certain patients who only feel better if the TSH is just above the reference range. It is recommended that each patient is treated as an individual and in conjunction with their supervising doctor is set a target that is right for them and their particular circumstances.

If you have been diagnosed with hypothyroidism you will start treatment with levothyroxine - a synthetic version of the thyroxine (T4) produced by the thyroid gland.

If you have hyperthyroidism the available treatments are antithyroid drugs to reduce the production of thyroid hormones; surgery to remove all or part of the thyroid gland; or radioactive iodine to reduce the activity of the thyroid. Your doctor will discuss treatment options with you.

At the start of treatment your doctor will carry out blood tests usually every few weeks. The results will help to fine-tune your treatment. You will normally have less frequent tests when you are stable on your treatment. In hypothyroidism, a TSH test once a year will check that levels are within the reference range. In hyperthyroidism the usual tests are TSH and FT4; how often these take place will depend on the treatment.

You will have additional tests if the results are abnormal, and you should tell your doctor about any change in your health between blood tests. If your results are normal, but you still don’t feel entirely well, ask your doctor whether there is room for a slight adjustment of your dose. This can be considered if your TSH level can be kept within the reference range. You should not, however, alter your dose without discussing this with your doctor.

Once you start on levothyroxine it may take several months before your symptoms improve even if the tests are biochemically satisfactory. This is especially the case in patients with a history of Graves’ disease who may have been hyperthyroid for many months and who may take a considerable time to adjust to feeling ‘normal’ with biochemically satisfactory tests following radioiodine or surgery.

Important points regarding Thyroid Tests: 

• Blood tests are currently the most accurate way to diagnose and manage thyroid disorders
• Your symptoms and how you feel are an important part of the diagnosis
• It is important for your health that the TSH level is within the reference range
• If you are taking medication for a thyroid disorder, there may be scope to fine-tune your treatment so that you feel better
• If you have a diagnosed thyroid disorder or have had previous treatment for an over-active thyroid, it is important to have a blood test every 12 months, or as advised by your doctor
• If you have a thyroid disorder you should have a blood test in early pregnancy or if you are planning a pregnancy
• If you are taking medication, do not alter your dose without discussing this with your doctor
• It is well recognised that thyroid problems often run in families and if family members are unwell they should be encouraged to discuss with their own doctors whether thyroid testing is warranted.

If you have questions or concerns about your thyroid disorder, you should talk to your doctor or specialist as they will be best placed to advise you. 

TSH TESTS
The best way to initially test thyroid function is to measure the TSH level in a blood sample. A high TSH level indicates that the thyroid gland is failing because of a problem that is directly affecting the thyroid (primary hypothyroidism). The opposite situation, in which the TSH level is low, usually indicates that the person has an overactive thyroid that is producing too much thyroid hormone (hyperthyroidism). Occasionally, a low TSH may result from an abnormality in the pituitary gland, which prevents it from making enough TSH to stimulate the thyroid (secondary hypothyroidism). In most healthy individuals, a normal TSH value means that the thyroid is functioning normally.

T4 TESTS
T4 circulates in the blood in two forms:

1) T4 bound to proteins that prevent the T4 from entering the various tissues that need thyroid hormone.

2) Free T4, which does enter the various target tissues to exert its effects. The free T4 fraction is the most important to determine how the thyroid is functioning, and tests to measure this are called the Free T4 (FT4) and the Free T4 Index (FT4I or FTI). Individuals who have hyperthyroidism will have an elevated FT4 or FTI, whereas patients with hypothyroidism will have a low level of FT4 or FTI.

Combining the TSH test with the FT4 or FTI accurately determines how the thyroid gland is functioning.

The finding of an elevated TSH and low FT4 or FTI indicates primary hypothyroidism due to disease in the thyroid gland. A low TSH and low FT4 or FTI indicates hypothyroidism due to a problem involving the pituitary gland. A low TSH with an elevated FT4 or FTI is found in individuals who have hyperthyroidism.

T3 TESTS
T3 tests are often useful to diagnosis hyperthyroidism or to determine the severity of the hyperthyroidism. Patients who are hyperthyroid will have an elevated T3 level. In some individuals with a low TSH, only the T3 is elevated and the FT4 or FTI is normal. T3 testing rarely is helpful in the hypothyroid patient, since it is the last test to become abnormal. Patients can be severely hypothyroid with a high TSH and low FT4 or FTI, but have a normal T3. In some situations, such as during pregnancy or while taking birth control pills, high levels of total T4 and T3 can exist. This is because the estrogens increase the level of the binding proteins. In these situations, it is better to ask both for TSH and free T4 for thyroid evaluation.

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario. For further information about these lab tests contact Symbion VIP Diagnostics pathology lab Ahmedabad at 09429410291