Table of Contents
- What is Thyroid Function Test?
- Thyroid Functions
- Thyroid Function Test Results Normal Ranges
- Thyroid Function Tests Result Interpretation
- Problems in interpretation of Thyroid Function Tests
What is Thyroid Function Test?
Thyroid Function Test is a type of Thyroid test that is done to ascertain or assess the thyroid function; it is also referred to as the Thyroid Profile Test. It helps to show if the thyroid cells are producing the required amount of Thyroid hormones. When the thyroid cells produce excessive amount of thyroid hormone, the thyroid function test will show increased blood level of T3(Triiodothyronine) and T4(Thyroxine) hormones, this condition will be termed Hyperthyroidism (Thyrotoxicosis) or an Overactive thyroid. When the thyroid cells become less active, the thyroid is termed Underactive Thyroid or Hypothyroidism and in such a situation, the thyroid hormones levels will be low. The Thyroid Function Test commonly assesses 3 parameters which are the T4, T3 and TSH (Thyroid Stimulating Hormone) level with TRH (Thyrotropin Releasing Hormone) in some special circumstances.
Functions of the Thyroid gland are diverse depending on the part of the body the hormones act on. The thyroid is responsible for producing the hormones that regulate metabolism in the body. These hormones have various effects in different parts of the body.
Thyroid Hormones Functions
- Thyroid hormone causes increased rate of heart beat (raised heart rate) and also causes increase cardiac output. This is the effect on the cardiovascular system.
- On the Skeletal system, the hormones cause increased bone turnover and resorption
- Thyroxine maintains the normal hypoxic and hypercapnic drive in respiratory center.
- The thyroid hormones increase the movement of the bowels, this is the reason why increase levels cause diarrhea as one of the symptom of thyrotoxicosis.
- Thyroxine also helps the body to increase the speed at which muscles contraction and relaxation and muscle protein turnover, hence helping us to respond quickly.
Thyroid Function Test Results Normal Ranges
Normal ranges of thyroid function tests include:
- Normal Free T3 level is from 3.5 to 7.5 micromol/L
- Normal Free T4 level is from 10 to 30 nmol/L
- Normal TSH level is from 0.3 to 3.3 mIU/L (Milli-International Unit per Litre)
Thyroid Function Tests Result Interpretation
Interpretation of the Thyroid Function Tests is done by comparing the levels of either T3 with TSH or T4 with TSH or both T3, T4 and TSH.
The TSH test results help to differentiate between Hyperthyroidism, Hypothyroidism or True Thyroid (Normal thyroid or Euthyroid). There are however, conditions that give false levels of TSH such as having false low TSH levels in thyroid diseases such as sick euthyroid syndrome and dysthyroid eye disease and others such as hypopituitarism. Because of these thyroid problems, TSH is not used as a single Thyroid Test even though it is sensitive but making an accurate diagnosis of thyroid disease requires addition of T4 and T3 levels to the Thyroid Profile to give a clear picture of how the thyroid is functioning.
Interpretation of TSH Test Results
- When TSH is high and T4 and T3 are low, it means the thyroid is underactive or it is called hypothyroidism. High TSH levels shows the body is sensing that Thyroid hormones are low and therefore tries to stimulate the thyroid to produce more; but since the thyroid function is abnormal, the thyroid fails to produce more of the hormones and the TSH keeps being secreted leading to more and more levels in the body. This is why TSH is high is hypothyroidism.
- When TSH is low and T4 and T3 levels are high, it means the thyroid is overactive and this is referred to as Hyperthyroidism. Low TSH levels means the body senses that T4 and T3 are high and so responds by reduced stimulation of Thyroid gland.
- When TSH level is slightly raised and T4 and T3 are normal, this is referred to as Subclinical Hypothyroidism.
T3 and T4 Results interpretation
- When T4 and T3 are raised and TSH is low, it is Hyperthyroidism
- When T4 and T3 are raised and TSH is high, it is Hypothyroidism
- Normal T4 and T3 and slightly raised TSH signifies Subclinical hypothyroidism
TRH test result is not often requested except for investigation of hypothalamicpituitary dysfunction. TRH (also called protirelin) is occasionally used to differentiate between thyroid hormone resistance and TSHoma (Thyrotropinoma or Thyrotropin Secreting Pituitary Adenomas) in the context of raised free T 4 and TSH levels. In differentiating between TSHoma vs Thyroid hormone Resistance, there is a rise in TSH in thyroid hormone resistance, whilst in TSHoma there is a flat response due to continued autonomous TSH secretion which does not respond to TRH stimulation test.
Problems in interpretation of Thyroid Function Tests
There are certain factors or disease conditions that may affect the Thyroid Function Tests Result Interpretation.
Drugs affecting interpretation of thyroid function test results
Some drugs can affect the levels of T4 and T3 in the body and give a false result leading to wrong diagnosis. Such drugs include Amiodarone which may give a false high level of T4 in a patient with normal thyroid function.
Pregnancy and use of Oral Contraceptive pills
The thyroid function test could be affected in pregnancy or in women using oral contraceptive pills as they both cause increase levels of Thyroxine Binding Globulin (TBG) which may lead to high Total level of T4 but normal level of Free T4 in the blood.
Serious acute and chronic systemic disease
Patients having systemic disease may have an apparently low levels of both total and free T 4/T 3 with a normal or low basal TSH, a condition called the sick euthyroid syndrome.