Thyroid – Part 5 – Thyroxine Total, T4 Total

Sample

1. This test is done on the serum of the patient.
2. The sample is stable for 7 days at room temperature but refrigeration at 4 °C is preferred.

Precaution

• Avoid lipemic serum.

Indications

1. This is done to assess thyroid function.
2. it is done to diagnose thyroid diseases.
3. This is also done to monitor the therapy.

Pathophysiology

1. Thyroxine T4 forms by the combination of :
   1. diiodotyrosine + diiodotyrosine = Tetra Iodo Tyrosine (Thyroxine or T4).

2. T4 makes most of the thyroid hormone because T3 is 10 %.
   1. 80% of the T4 is metabolized into:
      1. T3 = 35%
      2. rT3 = 45%
3. Most of the T4 is bound to thyroglobulin and albumin.
   1. T4  is pro-hormone with thyroglobulin binding (TBG).

4. There is a very small fraction of free T4.
   1. Serum Total T4 = bound T4 + Free T4

5. Thyroid glands have a major role in body function. T4 and T3 have a role in the growth, maturation of bone, and skeletal system.
   1. T3 is 3 to 8 times more metabolically active than T4 and is considered the active form of the thyroid hormone.
   2. It regulates body temperature.
   3. It has a role in the body’s metabolism of fats, carbohydrates, and proteins.

NORMAL

Source 1

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 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

Source 2

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
5to 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

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)

• Infants 1 to 3 days = 11 to 22  µg/dL.
  • Infants 1 to 4 months = 8 to 16 .µg/dL
• Child 1 to 5 years = 7 to 15 .µg/dL
  • Child 10 to 15 years = 5 to 12 µg/dL.
• Adult Male = 4 to 12  µg/dL.
  • Adult female = 5 to 12 µg/dL.
  • adult > 60 years = 5 to 11 µg/dL.

The increased T4 level is seen in:

1. Grave’s disease.
2. Toxic thyroid adenoma.
3. Struma ovarii.
4. Acute thyroiditis.

The decreased T4 level is seen in:

1. Hypothyroidism like:
   1. Cretinism.
   2. Myxedema.
   3. Surgical ablation.
2. Failure of the hypothalamus.
3. Protein-losing conditions like :
   1. Nephrotic syndrome.
   2. Renal failure.
4. Iodine insufficiency.
5. Cirrhosis.
6. Cushing’s syndrome.
7. Advanced cancers.
8. Pituitary insufficiency.

The critical value of T4 Total:

• Newborn = <7 µg/dL
• Adult = <2 µg/dL

Please see more details in Free T4 and thyroid function.

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