Levothyroxine absorption test

Investigation Protocol


To investigate unusual thyroid function tests eg persistantly elevated TSH despite apparently adequate levothyroxine replacement therapy. 


Patient may eat and drink, and take all medications as normal on the day of the test, though patients are asked to bring all their medication with them, for this to be documented at the start of the test. Iron and calcium containing supplements are known to affect the absorption of thyroxine.


Insert an indwelling cannula gauge 20 with a three-way tap. Using the vacutainer connector system or a syringe fill one plain clot bottle forTSH, free T4 and free T3 levels. This should be clearly marked with the patient’s details, date and time of sample.

Administer 700mcg levothyroxine orally with water(or other dose as prescribed by the referring physician). Take further samples for free T4 and TSH at 30, 60, 90 and 120 minutes. A fter each blood sample is taken, flush the cannula with 2mls sodium chloride. Prior to taking each blood sample, withdraw 2mls and discard to remove the sodium chloride flush. It is important to clearly mark patient’s details, date and time of sample on each bottle.

At end of test remove cannula and patient may go home.

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Patient information sheet

Levothyroxine absorption test

Your doctor has referred you to have a levothyroxine absorption test. This is performed over a two hour period to determine whether your body can absorb thyroxine after this is administered in tablet form. 

Please fast from midnight on the day of the test, but take all of your medications as normal with water only on the morning of the test. Please also bring all of your tablets with you to be checked and recorded at the start of the test. 

During the test blood samples will be taken regularly. To allow this to happen easily with the minimum of discomfort we will insert a small tube (a cannula) into a vein in your arm at the start of the test. We will then take a sample of blood at the start of the test, and repeat this every 30 minutes for 2hours after you have been given a dose of thyroxine. You should expect your stay at the hospital to last approximately 2 hours.

You may wish to bring a book or magazine to read. We do have a radio, or you can access Patientline; alternatively you may like to bring your own personal radio/cd player (please bring your own headphones). You may bring someone to stay with you during the test but there is not enough space for more than one person.

Children are discouraged, as this can be disruptive to other patients. However, if this poses a particular problem for you please discuss this with the senior nurse in the unit. You should be able to undertake normal activities after the test is finished and it should not interfere with your ability to drive home. 

If you have any queries about the test, please contact: 

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