Thyroid Function Tests

A healthy thyroid produces about 80 % of thyroxine (T4) and about 15 % of triiodothyronine (T3). It also produces calcitonin and some other smaller hormones such as diiodothyronine (T2) and monoiodothyronine (T1).

A thyroid function test (TFT) should include the following tests;

If the TSH is low then the free T4 will be high which is indicative of an overactive thyroid (hyperthyroidism).

If the TSH is high then the free T4 will be low which is indicative of an underactive thyroid (hypothyroidism).

However it is not that uncommon for people to have a TSH which is normal (even low normal) with a low normal free T4 (free T4 is at the lower end of the normal scale). In this case you can still be hypothyroid, especially if you have classic symptoms/signs, positive thyroid antibodies and family history of thyroid disease. Clearly in these cases there is not an optimal amount of thyroxine, yet the pituitary gland is not sensing this and therefore not stimulating the thyroid to produce more thyroid hormone.

Unfortunately the blood tests can also not show you what happens within the cells. The T4 (thyroxine) needs to be converted to T3, so if this conversion does not take place then the patient can still be hypothyroid at tissue level, despite normal blood test results.

Timing of the blood tests once you are on thyroid hormone replacement therapy

According to the European Journal of Endocrinology your peak level of free T4 (thyroxine) is approximately three hours after you take your thyroxine in the morning (a sign you have recently absorbed the thyroxine) and your lowest level of free T4 is just before your next dose.

You must take the timing of the blood test into consideration when interpreting your blood test results. If you take T3 (like Tertroxin) in addition to T4 then you must wait at least 3 to 4 hours before you do the blood test, because after you take your T3, your free T3 level will be elevated for a number of hours.

Alternatively you can do the blood test before you take your medication, so you know what your lowest level is. If you have just started on thyroid hormone replacement therapy or you have changed your dose, then you should be re-tested 6 weeks later.

It is a good idea to;

Aim for optimal thyroid hormone levels (free T4 and free T3 should be in the upper half of the normal reference range).

The tests results must be used as a guide only and should not replace clinical evaluation!

The following should be considered for the diagnosis and treatment of autoimmune thyroid disease;

Medicare

The item number for thyroid function test, comprising of TSH, free T4 and free T3 is 66719. The TSH test is covered by Medicare, but the free T4 and the free T3 will only be paid by Medicare if at least one of the following conditions is satisfied (see MBS schedule below).

However if one of the following conditions is NOT satisfied, you can request for private billing. The laboratory should test the free T4 and/or the free T3 if the tests have been requested on the referral form by the doctor and it is clear you will be paying for the tests.

If the doctor writes down TFT then the laboratory may only test the TSH and the free T4. If the doctor writes down TSH, free T4 and free T3 separately on the form then there will be no confusion and the proper tests will be done.

For more information on item numbers you can check online at www.mbsonline.gov.au under MBS.

eg.Item 66719 - Thyroid function tests

“Thyroid function tests (comprising the service described in item 66716 and 1 or more of the following tests – free thyroxine, free T3, for a patient, if at least 1 of the following conditions is satisfied:
(a) The patient has an abnormal level of TSH;
(b)The tests are performed:
           (i) for the purpose of monitoring thyroid disease in the patient; or
           (ii) to investigate the sick euthyroid syndrome if the patients is an admitted patient; or
           (iii) to investigate dementia or psychiatric illness of the patient; or
           (iv) to investigate amenorrhoea or infertility of the patient;
(c) the medical practitioner who requested the tests suspects the patient has pituitary dysfunction;
(d) the patient is on drugs that interfere with thyroid hormone metabolism or function (item is subject to rule 9)”
This schedule from the MBS was up to date in May 2015.

Please read this article in conjunction with article Thyroid Stimulating Hormone.

Copyright © 2011-2015 Monique Atkinson

Disclaimer The information provided is for educational purposes only and is not intended to be medical advice. The contents must not be relied upon in place of advice and treatment from a qualified medical practitioner. THYROID WA SUPPORT GROUP INC. and the author disclaim any liability whatsoever.