Thyroid – Part 6 – Thyroxine T4, Free T4
Sample
- This test is done in the serum.
- Immediately refrigerate the serum.
- The sample is stable at room temp. for 7 days but refrigeration is better.
Precaution
- 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 will have raised the free T4 level.
Indications
- The FT4 level is used to evaluate thyroid function in a patient who has protein abnormality.
- It is used to diagnose thyroid function.
- This is done to monitor replacement and suppressive therapy.
Pathophysiology
- Thyroid hormone is made of 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 TBG and albumin).
- Only 0.04% is free T4 and this is biologically active.
- Only 1 to 5 % 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 be also 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 as a result of hypothyroidism.
- Total T4 is a good indicator of thyroid function if the TBG is normal.
Normal
Source 1
T4 Free (FT4)
Age | ng/dL |
Newborn 1 to 4 days | 2.2 to 5.3 |
Child (2 weeks to 20 Years | 0.8 to 2.0 |
Adult 21 to 87 years | 0.8 to 2.7 |
Pregnancy | |
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)
Age | µg/dL |
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 |
Source 2
T4 Free
- 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)
Age | µg/dL |
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 |
The increased level is seen in:
- Grave’s disease.
- Hypothyroidism treated with thyroxine.
- Euthyroid sick syndrome.
- Hyperthyroidism.
- Toxic thyroid adenoma.
- Acute thyroiditis.
- Struma ovarii.
The decreased level is seen in:
- Primary hypothyroidism.
- Secondary hypothyroidism (pituitary).
- Congenital TBG deficiency.
- Tertiary hypothyroidism (Hypothalamic).
- Cushing’s disease.
- Cirrhosis.
- Renal failure.
- Iodine insufficiency.
- Myxedema.
- Advanced cancer.
- Please see more details on the Thyroid function.