Thyroid – Part 1 – Thyroid Function Test (Thyroid Hormones) T4, T3, TSH

September 27, 2020Chemical pathologyLab Tests

Sample

1. The serum of a patient is needed.
2. TSH sample is stable 5 to 7 days at 4 °C.
3. T4 and T3 sample is stable for 7 to 14 days at room temperature (37 °C) or at 2 to 8 °C.

Precautions

1. Sample with hemolysis or lipemia is rejected.
2. Centrifuge the turbid sample before performing the test.

Thyroid functions tests advised are:

1. T3 uptake.
2. Free T3.
3. Total T3.
4. Total T4.
5. Free T4.
6. Free thyroxine index.
7. TSH.
8. Thyroglobulin (Tg)

Indications

1. These tests are done to evaluate thyroid dysfunction.
2. T3 is advised to diagnose hyperthyroidism.
3. T3 and T4 are advised to monitor replacement and suppressive therapy.
4. Free T4 is advised to evaluate the thyroid function in a patient with protein abnormality which will affect total T4.
5. The thyroid function test differentiates between :
   1. Euthyroid (thyroid function is normal).
   2. Hyperthyroidism (Increased thyroid function).
   3. Hypothyroidism (decreased thyroid function).

Pathophysiology

1. Thyroid follicles are the basic secretory unit of the thyroid gland.
2. The follicle has an outer layer of the epithelial cell, surrounding the amorphous colloid material.
3. Colloid mainly consists of thyroglobulin and a small amount of thyroalbumin.

Synthesis of the Thyroid Hormone

Thyroxine as a carrier protein

Thyroglobulin is a carrier protein

4. The thyroid gland has another cell known as parafollicular cell or C cell present within the follicular basement or interfollicular spaces. These cells produce a hormone known as Calcitonin.
   1. The thyroid gland produces thyroxine (T4) and Triiodothyronine (T3).
   2. Diiodothyronine is the precursor of T3 and T4.

TSH functions

5. T4 changes into T3 and reverse T3 (rT3) by deiodination.

Thyroxine and its biological forms

Functions of the thyroid:

1. control energy expenditure of the body.
2. Stimulate heart contraction and heart rate.
3. Stimulate carbohydrate and protein metabolism.
4. Increase in the synthesis and degradation of cholesterol and triglycerides.
5. It increases the sensitivity of the beta-adrenergic receptor to catecholamine.

Thyroid gland functions

Effect of increased thyroid hormones

6. Pituitary thyroid-stimulating hormone (TSH) regulates the synthesis of thyroid hormone.

Target organs for the Thyroid hormones

The pituitary gland and its effect on the thyroid gland

TSH induces:

1. Increase in the size and number of thyroid follicles.
2. Prolonged TSH stimulation leads to increased vascularity and may form a goiter.
3. The thyroid hormone maintains the rate at which your body uses fats and carbohydrates.
4. Control your body temperature.
5. Regulate the production of protein.
6. The thyroid gland also produces calcitonin, a hormone that helps regulate the amount of calcium in your blood.

The cycle of thyroid hormone formation:

1. The rate at which T3 and T4  are released and is controlled by the pituitary gland and hypothalamus (acts as a thermostat).
   1. The hypothalamus signals the pituitary gland it makes Thyroid-stimulating hormone (TSH).
   2. The pituitary gland is a source of TSH.
   3. The amount of TSH depends on the amount of T3 and T4 in blood.
   4. There is a feedback mechanism. If there is a decreased level of T3 and T4  TSH level will increase.
2. If T3 and T4 are increased then the TSH level will fall below normal.
3. The thyroid gland regulates its production of T3 and T4 based on the amount of TSH it receives.

Thyroid hormones regulatory control:

1. Hypothalamus produces TRH + Somatotropin  stimulate the Pituitary gland to produce TSH
2. TSH  acts on the thyroid gland stimulate secretion of T4 + iodine trapping + new hormone synthesis + stimulate thyroid cell hypertrophy (Goiter).
3. T4 + T3 = negative feedback mechanism on pituitary gland + hypothalamus.

TSH feedback effect on the pituitary gland

4. Hypothyroidism when the thyroid does not produce enough T3 and T4.
5. Hyperthyroidism when thyroid glands produce an excess of T3 and T4.

TSH role in the regulation of T3 and T4

Metabolism of T3 and T4:

1. T3 and T4 conjugate in the liver to form Sulphate and glucuronide.
2. These conjugates enter the bile and to the intestine.
3. Thyroid hormone conjugates are hydrolyzed and reabsorbed from the intestine (Enterohepatic circulation) and some are excreted in the stool.
4. The T3 rise may be the first and only abnormality in the case of Hyperthyroidism.

T3 and T4 metabolism

• The thyroid function tests are depended upon various factors.

Thyroid functions

Normal Values

Source 1

T3 Total

• Cord blood  (>37 weks) = 5 to 141 ng/dL
• 1 to 3 days = 100 to 740 ng/dL
• 1 to 11 months = 105 to 245
• Children
  1. 1 -5 years = 105  to 269 ng/dL
  2. 6 to 10 years = 94 to 241 ng/dL
  3. 16 to 20 years = 80 to 210 ng/dL
• Adult
  1. 20 to 50 years = 70 to 204 ng/dL
  2. 50 to 90 years = 40 to 181 ng/dL
• Pregnancy
  1. Last 5 months = 116 to 247 ng/dL
     1. To convert into SI unit x 0.0154 = nmol/L

Source 1

Free T3 

• Cord blood (>37 weeks ) = 15 to 391 pg/dL
• Child and adult = 260 to 480 pg/dL  (4.0 to 7.4 pmol/L)
• Pregnancy
  • First trimester = 211 to 383 pg/dL
  • Second and third trimester = 196 to 338 pg/dL

Source 2

T3 total

• 1 to 3 days = 100 to 740 ng/dL
• 1 to 11 months = 105 to 245 ng/dL
• 1 to 5 years = 105 to  270 ng/dL
• 6 to 10 years = 95 to 240 ng/dL
• 11 to 15 years = 80 to 215 ng/dL
• 16 to 20 years = 80 to 210 ng/dL
• 20 to 50 years = 75 to 220 ng/dL
• >50 years = 40 to 180 ng/dL

Source 1

T4 Total (T4)

Source 1

T4 Free (FT4)

• To converts SI unit x 12.9 = nmol/L

Source 2

T4 Total (T4)

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)

T3 and T4 and their biological forms

TSH

Source 1

• To converts into SI unit x 1.0 = mU/L

Source 2

• Adult = 0.4 to 5.6 mIU/L
• Newborn = 3 to 20 mIU/L
• Cord blood = 3 to 12 µU/mL
  • Values vary between laboratories.

Another source

• Adult
• 21 to 54 years = 0.4 to 4.2 mU/L
• 55 to 87 years = 0.5 to 8.9 mU/L
• Newborn = 1.7  to 9.1 mU/L
  • 0.3 to 3.04 mIU/L  (another reference)
• Pregnancy
  • First trimester = 0.3 to 4.5 mU/L
  • Second trimester = 0.5 to 4.6 mU/L
  • Third trimester = 0.8 to 5.2 mU/L

(Normal values varies from lab to lab and different methodology)

Hypothyroidism, There is inadequate production of thyroid hormones:

1. It is characterized by decreased T4.
2. Ther is an increased level of TSH.
3. Signs and Symptoms of Hypothyroidism
   1. The signs and symptoms of hypothyroidism vary widely, depending on the severity of the hormone deficiency.
   2. At first, you may barely notice symptoms such as fatigue and sluggishness.
   3. Increased sensitivity to cold
   4. Constipation.
   5. Skin is pale and dry skin.
   6. There is puffiness of the face due to edema.
   7. The patient develops Hoarseness of voice.
   8. There is an increased level of cholesterol.
   9. The patient also gains weight.
   10. There are Muscle aches, tenderness, and stiffness.
   11. There is Muscle weakness
   12. There may be pain, stiffness, or swelling of joints.
   13. Female patients develop heavier menstrual periods.
   14. The patient may develop signs of Depression.
   15. Usually, these patients become obese.
   16. There may be joint pain.
   17. This patient may suffer from infertility.
   18. These patients may develop heart problems.
   19. In untreated cases, signs and symptoms become worse.
   20. Constant stimulation of the thyroid gives rise to enlargement of the gland, which is called Goiter.
   21. Advanced hypothyroidism is known as Myxedema.
4. Myxedema includes more signs and symptoms like :
   1. Low blood pressure.
   2. Decreased breathing.
   3. Decreased body temperature.
   4. Unresponsiveness and even the patient may go into a coma.
   5. Myxedema may be fatal in some patients.
5. Causes of hypothyroidism
   1. Autoimmune thyroid disease.
   2. Iodine deficiency.
   3. iodine-induced.
   4. Goitrogen exposure.
   5. A defect in hormone synthesis.
   6. Thyroid ablation by surgery or radiation.
   7. Thyroid agenesis or dysgenesis.
   8. Hypothalamic-pituitary disease.

Thyroid function in Hypothyroidism:

Hyperthyroidism (Thyrotoxicosis), There is excessive production of thyroid hormones:

1. It is characterized by elevated T4.
2. There is a decreased or even absent level of  TSH.
3. Signs and Symptoms of Hyperthyroidism
   1. Hyperthyroidism can significantly accelerate your body’s metabolism, causing :
   2. Sudden weight loss, even appetite, and food intake remain normal or increase.
   3. Rapid or irregular heartbeat ( Tachycardia, Palpitation may be more than 100 beats/minutes.).
   4. The patient may even develop Arrhythmias.
   5. The patient develops sweating.
   6. Nervousness or irritability.

   Graves disease This is an autoimmune disorder.

   1. Grave’s Disease is due to Hyperthyroidism.
   2. symptoms of hyperthyroidism:
      1. Tremor, these are usually fine trembling with hands and fingers.
      2. Patients have increased sensitivity to heat.
      3. Changes in bowel patterns, especially more frequent bowel movements.
      4. An enlarged thyroid gland (goiter).
      5. The patient will have Fatigue, muscle weakness.
      6. The patient will have sleep difficulty.
   3. Beta-blockers can mask many of the symptoms of hyperthyroidism
4. Causes of hyperthyroidism
   1. Multinodular goiter.
   2. Single toxic goiter.
   3. Subacute thyroiditis.
   4. Autoimmune thyroid disease.
   5. Pituitary adenoma (thyrotropin producing tumor).
   6. Gestational trophoblastic tumor.
   7. Postpartum Thyroid disease.
   8. Hyperemesis gravidarum.
   9. Exogenous thyroid hormones.
   10. Metastatic thyroid cancer.
   11. iodine-induced.

TSH role in various diseases

Thyroid functions in Hyperthyroidism:

Table showing lab. test in Thyroid diseases:

Goiter

1. Definition This is an enlarged thyroid gland because of any cause.
2. Causes are :
   1. Inflammatory.
   2. A function that may be normal increased or decreased.
   3. Neoplastic.
3. Most of the goiter has normal function seen in 90 % of the multinodular goiter and these are almost all are a colloid goiter.
4. Euthyroid sick syndrome This is also known as low T3 syndrome
   1. There is biochemical hypothyroidism as T4 is shifted away to form T3.
   2. Total T4 and TSH usually remain within normal limits.

Thyroid function tests Summary:                       

• please see more details in T3 and T4.