What is a standard thyroid function test?

Thyroid function test

A thyroid function test is performed to assess how well your thyroid is working.

Thyroid stimulating hormone (TSH) is accepted as the first line test for assessment of thyroid function. When there is a deficiency of thyroid hormones, the pituitary gland releases more TSH to signal the thyroid gland to produce greater quantities of thyroxine (T4) and triiodothyronine (T3).

TSH is just one blood test that doctors use to assess thyroid status as part of a thyroid function test. Other blood tests measure the actual amount of thyroid hormones found in the blood.

If an underactive thyroid gland is suspected, blood tests should not be limited to just TSH. Other useful tests should include thyroid antibodies, free T3, free T4 and reverse T3.

Another method is the ‘Basal body temperature’ (BBT). The temperature  is taken at rest – preferably first thing in the morning before rising. A normal BBT is 36.5°C and above. Progesterone produced after ovulation raises BBT by counteracting the thyroid-suppressing effect of oestrogen. So for this reason, it is recommended for menstruating women to conduct the test on the second, third and fourth day of their period.

Although a low BBT may be due to other causes such as low adrenal hormones, it does warrant further investigation with a thyroid function test and in-clinic testing.

Your thyroid is a very complex gland. If you suspect your thyroid gland may be underactive as per the symptoms list, it is recommended you have a blood test to check your thyroid hormones.

The problem with current reference ranges

The TSH reference range used by many laboratories is between 0.2-5.5 (mU/L). A greater TSH number is indicative of a thyroid hormone deficiency. That is because the pituitary is over-releasing TSH based on a lack of thyroid hormone in the blood. A reading of more than 5.5 alerts a doctor to a thyroid gland problem and that thyroid hormone therapy may be warranted.

The American National Academy of Clinical Biochemistry narrowed the window of normal for thyroid stimulating hormone (TSH) from 0.5-5 to 0.2-2.5mIU/L. Similar revisions by the American Association of Clinical Endocrinologists (AACE) meant that 13 million people previously considered normal could become officially diagnosed with underactive thyroid.

If you have depression, heart disease, high cholesterol, chronic fatigue, poor mental performance or any of the other symptoms associated with low thyroid activity, you may want to ask your doctor to ‘defy the reference ranges’ and try different thyroid replacement therapeutic approaches. Your naturopath will also be able to assist with natural treatment for thyroid gland problems.

If you suspect low thyroid function, but have had ‘normal’ blood tests, speak to your naturopath about natural solutions to boost thyroid gland activity.

Basal body temperature testing

Shake the thermometer well so that the mercury returns to the reservoir at the base. Place the thermometer on your bedside table, within easy reach while you are in bed.

In the morning, immediately on waking and before you get out of bed, place the thermometer under your arm, well into the armpit. Leave it there for a full three minutes. Then record you reading.

For menstruating women, it is important to measure your temperature during menstruation. For example, measure your temperature on days 2, 3 and 4 of your period. The temperature should be between 36.4-36.6°C.

If your basal temperature is below this level for three mornings in succession, it likely means that your basal (resting) metabolic rate is low and indicates an under active or ‘sluggish’ thyroid.

It is important not to test your temperature when you have an infection or any other condition which may raise your temperature.