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Thyroid:- Part 5 – Thyroxine Total, T4 Total

March 28, 2023Chemical pathologyLab Tests

Table of Contents

  • Thyroxine (T4 Total)
      • Sample for T4 Total
      • Precaution for Thyroxine Total (T4 Total)
      • Indications for Thyroxine Total (T4 Total)
      • Definition of Thyroxine Total (T4 Total)
      • Pathophysiology of thyroid hormones
      • Thyroxine (T4) interpretations:
      • Thyroxine (T4) and triiodothyronine (T3) functions:
        • NORMAL  Thyroxine (T4)
      • T4 Total (T4)
        • Source 1
      • T4 Free (FT4)
      • T4 Total (T4)
        • Another Source 2
      • T4 Free
      • The increased T4 level is seen in the following:
      • The decreased T4 level is seen in the following:
      • Questions and answers:

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).
Thyroglobulin role in the synthesis of T3 and T4

Thyroglobulin’s role in the synthesis of T3 and 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).
Thyroxine carrier proteins

Thyroxine carrier proteins

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

Thyroxine formation

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.
Thyroid hormones T4 and T3 functions

Thyroid hormones T4 and T3 functions

  • In the case of raised thyroid hormones, T4 and T3 lead to signs and symptoms in the patient.
Increased Thyroid hormones

Increased Thyroid hormones

NORMAL  Thyroxine (T4)

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
 Adults 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

Another source 2

T4 Total (T4)

Age µg/dL
1 to 3 days 11.22
1 to 2 weeks 10 to 16
One o 12 months 8 to 16
1 to 5 years 7 to 15
5 to ten 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

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:

Question 1: What is thyroxine?
Show answer
Thyroxine is also called as T4. It includes free and bound T4.
Question 2: Which thyroid hormone is biologically active?
Show answer
T4 is converted to T3 and free T3 is biologically active.

Please see more details on Free T4 and thyroid function.


Possible References Used
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