Thyroid:- Part 6 – Thyroxine T4, Free T4
March 28, 2023Chemical pathologyLab Tests
Free T4 (FT4)
Sample for Free T4
- This test is done in the serum.
- Immediately refrigerate the serum.
- The sample is stable at room temperature for seven days, but refrigeration is better.
Precaution for Free T4
- Avoid lipemic serum.
- Neonates have a higher level than older children and adults.
- Some of the drugs increase T4 levels, like heparin, aspirin, propranolol, and danazol.
- Some of the drugs decrease the T4 level, like furosemide, rifampicin, phenytoin, and methadone.
- If a patient taking exogenous thyroxine has raised the free T4 level.
Indications for Free T4
- The Free T4 level is used to evaluate thyroid function in a protein abnormality patient.
- It is used to diagnose thyroid function.
- This is done to monitor replacement and suppressive therapy like levothyroxine. TSH takes 6 to 8 weeks to show changes.
- Free T4 gives corrected values in patients where the total T4 is changed on account of changes in serum proteins or in binding sites like:
- Drugs such as androgens, birth control pills, estrogens, and phenytoin.
- In pregnancy.
- In the case of nephrosis, where serum proteins are altered.
- Usually not helpful unless hypothalamus/pituitary disease is suspected.
Pathophysiology of thyroid hormones
- The thyroid hormone comprises Triiodothyronine (T3) and Thyroxine (T4).
- Over 90 % of thyroid hormone is made of Thyroxine T4.
- As much as 99 % of T4 is bound to protein (Thyroid binding globulin and albumin).
- Only 0.04% is free T4, and this is biologically active.
- Only 1 to 5% of T4 is unbound or free T4 (source 2).
- Free thyroxine FT4 comprises a small fraction of total thyroxine.
- When measuring total T4 = unbound T4 + bound T4.
- The protein level influences the T4 level.
- FT4 is unbound to protein and is available to the tissue.
- FT4 is a metabolically active hormone.
- Pregnancy and hormone replacement therapy increase the TBG and lead to false increases at the T4 level.
- In that case, suggest hyperthyroidism when actually the patient is euthyroid.
- In such cases, when you measure Free T4, it will be normal, indicating the measurement of the Free T4 is more accurate than the total T4.
- In the case of hypoproteinemia, TBG is low, so total T4 will also be low, suggesting hypothyroidism.
- Now if you measure Free T4, it will be the normal level and negate the abnormal level of total T4, which is due to a low level of TBG and not a result of hypothyroidism.
- Total T4 is a good indicator of thyroid function if the TBG is normal.
Normal T4 Free (FT4)
T4 Free (FT4)
|Newborn 1 to 4 days||2.2 to 5.3|
|Child (2 weeks to 20 Years||0.8 to 2.0|
|Adults 21 to 87 years||0.8 to 2.7|
|1st trimester||0.7 to 2.0|
|2nd and 3rd trimester||0.5 to 1.6|
- To converts SI unit x 12.9 = nmol/L
T4 Total (T4)
|Cord blood||7.4 to 13.0|
|1 to 3 days||11.8 to 22.6|
|1 to 2 weeks||9.8 to 16.6|
|1 to 4 month||7.2 to 14.4|
|4 to 12 month||7.8 to 16.5|
|1 to 5 year||7.3 to 15.0|
|5 to 10 year||6.4 to 13.3|
|10 to 15 year||5.6 to 11.7|
|Adult||Male = 4.6 to 10.5 Female = 5.5 to 11.0
|>60 year||5.0 to 10.7|
|Maternal serum 15 to 40 weeks||9.1 to 14.0|
Another source 2
- Newborn (0 to 4 days) = 2 to 6 ng/dL (26 to 77 pmol/L)
- Child 2 weeks to 20 years = 0.8 to 2.0 ng//dL
- Adult = 0.8 to 2.8 ng/dL (10 to 36 pmol/L)
Newborn FT4 index
- Newborn FT4 index = 9.9 to 7.5 ng/dL
- pubertal child and adult FT4 index = 4.2 to 13.0 ng/dL
T4 Total (T4)
|1 to 3 days||11.22|
|1 to 2 weeks||10 to 16|
|1 o 12 months||8 to 16|
|1 to 5 years||7 to 15|
|5 to 10 years||6 to 13|
|10 to 15 years||5 to 12|
|Adult||Male 4 to 12 Female 5 to 12|
|>60 years||5 to 11|
Free T4 (FT4) increased level is seen in the following:
- Grave’s disease.
- Hypothyroidism is treated with thyroxine.
- Euthyroid sick syndrome.
- Toxic thyroid adenoma.
- Acute thyroiditis.
- Struma ovarii.
- Patients with hydatidiform mole or choriocarcinoma where the HCG level is raised will show:
- Increased Free T4.
- Suppressed TSH.
- Blunted response of TSH response to TRH stimulation.
The decreased Free T4 level is seen in the following:
- Primary hypothyroidism.
- Hypothyroidism is treated with triiodothyronine.
- Secondary hypothyroidism (pituitary).
- Congenital TBG deficiency.
- Tertiary hypothyroidism (Hypothalamic).
- Cushing’s disease.
- Renal failure.
- Iodine insufficiency.
- Advanced cancer.
Questions and answers:
Question 1: What is the value of Free T4?
Free T4 is helpful in patients with abnormal serum proteins.
Question 2: What will happen to Free T4 in the Grave's disease?
Free T4 will be raised in Grave's disease.
- Please see more details on Thyroid function.