Trip Answers

Question answered:29/06/07 Warning! this question is over two years old.

The Association for Clinical Biochemistry’s guideline on the use of thyroid function tests notes on page 15:

 

“The earliest biochemical abnormality in hypothyroidism is an increase in serum TSH concentration associated with normal serum FT4 and FT3 concentrations (subclinical hypothyroidism), followed by a decrease in serum FT4 concentration, at which stage most patients have symptoms and benefit from treatment (overt hypothyroidism).” [1]

 

A second source, an e-Medicine article on hypothyroidism states:

 

“Localized disease of the thyroid gland that results in decreased thyroid hormone production is the most common cause of hypothyroidism. Under normal circumstances, the thyroid releases 100-125 mcg of thyroxine (T4) daily and only small amounts of triiodothyronine (T3). The half-life of T4 is approximately 7-10 days. T4, a prohormone, is converted to T3, the active form of thyroid hormone, in the peripheral tissues. Early in the disease process, compensatory mechanisms maintain T3 levels. Decreased production of T4 causes an increase in the secretion of TSH by the pituitary gland. TSH stimulates hypertrophy and hyperplasia of the thyroid gland and thyroid T4-5'-deiodinase activity. This, in turn, causes the thyroid to release more T3.” [2]

 

In answering this question, we consulted our Medical Director who said we should consider the significance of a low T3 with a normal T4 and TSH concentrations. He referred us to the Lab Tests Online website which offers the following information on interpreting thyroid test results:

 

 

The site adds:

 

“Many medications—including estrogen, certain types of birth control pills, and large doses of aspirin—can affect total T3 test results, so tell your doctor about any drugs you are taking. In general, free T3 levels are not affected by these medications. 

 

When you are sick, your body decreases production of T3 from T4. Most people who are sick enough to be in the hospital will have a low T3 or free T3 level. For this reason, doctors do not usually use T3 as a routine thyroid test for patients in hospitals.” [3]

 

GPNotebook’s entry on diagnosing hypothyroidism also questions the use of measuring T3 stating:

 

“free or total serum T3 is generally unhelpful since it may be only slightly reduced in severe hypothyroidism, or may be elevated by illness unrelated to thyroid function, or as a consequence of incorrect drug therapy.” [4]

 

References
1. Association for Clinical Biochemistry. UK guidelines for the use of thyroid function tests. July 2006. (http://www.acb.org.uk/docs/TFTguidelinefinal.pdf).
2. Orlander R and Woodhouse W. Hypothyroidism. e-Medicine. September 2005. (http://www.emedicine.com/med/topic1145.htm).
3. Lab Tests Online. T3: the test. Last updated July 2005. (http://www.labtestsonline.org/understanding/analytes/t3/test.html).
4. GPNotebook. Investigations in hypothyroidism. (http://www.gpnotebook.co.uk/simplepage.cfm?ID=255131657&linkID=19020&cook=yes

 

---

DISCLAIMER: TRIPanswers is a collection of clinical questions and answers.  Each provider will have their own methodology in answering questions and these are likely not to be as rigorous as systematic review.  If you have any doubt as to the implications of this contact the Q&A Service Provider for further information. This document is presented for information purposes only. This document cannot and should not be used as a basis of diagnosis or choice of treatment, and is in no way intended to replace professional medical care or attention by a qualified practitioner. TRIPanswers and TRIP Database Ltd are not responsible or liable for, directly or indirectly, ANY form of damage whatsoever resulting from the use/misuse of information contained in or implied by this document.  Also, ensure you have read the terms and conditions for using the site.