Thyroxine (T4 Total)

Sample for T4 Total

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 for Thyroxine Total (T4 Total)

• Avoid lipemic serum.

Indications for Thyroxine Total (T4 Total)

1. This is done to assess thyroid function.
2. It is advised to diagnose thyroid diseases like hypo or hyperthyroidism.
3. This is also done to monitor the therapy.

Definition of Thyroxine Total (T4 Total)

1. Thyroxine is also known as T4. Thyroxine is the major secretion of the thyroid gland bound to thyroid-binding globulin, prealbumin, and albumin.
2. Thyroxine is produced in the thyroid gland and is stimulated by the TSH hormone.
3. In tissue, thyroxine (T4) is deiodinated into T3, which causes hormonal action.
4. Thyroxine is needed for the body’s growth, body metabolism, and energy.
5. Thyroxine (T4) is synthesized in the thyroid gland by the amino acid tyrosine and iodine.

Pathophysiology of thyroid hormones

1. Thyroid hormone formation is basically intrathyroidal iodine metabolism. This process consists of the following stages:
   1. Iodine trapping is an uptake of iodine by the follicular cells.
   2. Oganification or iodination is the process where iodine is incorporated into thyroid hormones.
      1. Normally thyroid organifies about 75 µg of iodine per day.
   3. Coupling.
   4. Storage.
   5. Secretion.
2. Thyroxine T4 forms by the combination of :
   1. diiodotyrosine + diiodotyrosine = Tetra Iodo Tyrosine (Thyroxine or T4).

Thyroxine (T4) interpretations:

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

3. There is a very small fraction of free T4.
   1. Serum Total T4 = Bound T4 + Free T4
4. When T4 converts into T3, then hormonal action starts.

Thyroxine (T4) and triiodothyronine (T3) functions:

1. Thyroid glands have a major role in body function. T4 and T3 have a role in the growth and maturation of bone and skeletal systems.
   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 plays a role in the body’s fats, carbohydrates, and proteins metabolism.

NORMAL  Thyroxine (T4)

Source 1

T4 Total (T4)

Source 1

T4 Free (FT4)

• To converts SI unit x 12.9 = nmol/L

Another source 2

T4 Total (T4)

Another 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 the following:

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

The decreased T4 level is seen in the following:

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

Questions and answers:

Please see more details on Free T4 and thyroid function.