The thyroid absorbs the majority of iodine obtained from diet, from the blood. Iodoine is a mineral occuring naturally in food products such as fish and milk.
Image of the thyroid hormone system
Image by Mikael Häggström who has released it into the public domain
Iodine once absorbed and with tyrosine (an amino acid) undergoes a series of chemical reactions to produce T3
(triiodothryonine) and T4 (thyroxine).
TSH (thyroid stimulating hormone) is released from the pituitary gland in response to the TRH (thyrotropin-releasing hormone) from the hypothalamus. The hypothalamus acts via a feedback system, sensing when T3 and T4 levels are low, it produces TRH, secreting it to the pituitary gland which is stimulated to produce TSH. T4 is the main hormone produced by the thyroid, with T3 mostly being the result of a conversion in areas away from the thyroid.
TSH is released by the pituitary gland into the bloodstream. TSH reaches the thyroid and stimulates thyroid cells to produce T3 and T4. T3 and T4 act on all cells of the body, affecting the metabolic rate of the cells.
The levels of T3 and T4 feedback to the hypothalamus. High circulating T3 and T4 inhibit the hypothalamus secretion, whereas low levels stimulate the hypothalamus.
Thyroid Function Tests
Image of blood samples for testing
Image copyright of Shannan Muskopf of Flickr available under the Creative Commons license
Thyroid function tests are blood tests to assess the level of thyroid hormones discussed above in the blood.
T3 and T4 tend to be bound in blood to proteins, often thyroxine-binding globulin (TBG). Bound T3 and T4 tend to be inactive. Unbound and active T3 and T4 are referred to as free T3 (FT3) and free T4 (FT4) are levels of thyroid hormone. FT3 and FT4 are therefore important measurements and offer a more realistic view of thyroid function/dysfunction.
Total T4 (TT4) and toal T4 (TT4) measure bound and free levels of the relevant hormones.
The following table shows the normal references ranges that will be used to asses thyroid activity. Ranges for thyroid functions vary across laboratories, the ones used here are taken from UK Guidelines for the Use of Thyroid Function Tests.
|Thyroid Hormone||Normal Reference Range|
|TSH||0.4 – 4.5 mU/L|
|TT3||1.2 – 2.6 nmol/L|
|FT3||3.5 – 7.8 nmol/L|
|TT4||60 – 160 nmol/L|
|FT4||9.0 – 25 pmol/L|
Here mU/L = milliunits per litre (10-3 moles), nmol/L = nanomoles per litre (10-9 moles) and pmol/L =picomoles per litre (10-12)
A mole is unit of measure to assess the quantity of a chemical/substance.
What do the tests mean?
Thyroid function tests alone aren't sufficient to diagnose an disorder. Signs and symptoms also are important in diagnosis and should always be made by a qualified doctor.
Often in the case of hyperthyroidism TSH will be significantly reduced (<0.4mU/L) and levels of free and total T4 and T3 increased.
In hypothyroidism levels of T3 often aren't important, whereas TSH and T4 are. A TSH level >10 mU/L and raised TT4 and FT4 are indicitive of hypothyroidism.
For a raised TSH between 4.5 - 10 mu/L and normal T4 this is classed as subclincial hypothyroidism. This should be monitored via blood tests and if any symptoms of hypothyroidism occur (see symptoms) tested for earlier than the next sechduled thyroid function test.