Thyroid – Part 2 – Thyroid stimulating hormone (TSH), Thyrotropin

Sample

• Venous blood is taken and the test is done on the serum of the patient.

Indications

1. TSH differentiates Hypothyroidism from hyperthyroidism.
2. TSH used to diagnose primary hypothyroidism.
3. TSH used to differentiate primary from secondary hypothyroidism.
4. TSH also differentiates hypothyroidism due to the hypothalamus (Tertiary hypothyroidism).
5. TSH level is also done for the suppressive or replacement therapy.
6. TSH also has been done in a newborn with primary hypothyroidism who has a low T4 level.

Pathophysiology

1. TSH is a glycoprotein hormone composed of two polypeptide chains alpha and beta.
   1. It arises from the prehormone and prohormone.
   2. It has a short half-life of minutes to a few hours as compared to other hormones like steroids.
   3. TSH binds with the specific receptors for TSH found in the cytoplasmic membrane of the thyroid cells.
   4. It releases the second messenger that activates cAMP and protein kinase and stimulates further biochemical reactions.
   5. TSH is transported free and not bound to protein carrier in the body fluids.
   6. TSH with other adrenergic neuropeptides binds to the cytoplasm membrane receptors and the thyroid gland secretes hormone T4 and T3.
2. The thyroid gland has large stores of hormones and has a slow rate of normal turnover.

3. The pituitary gland produces TSH which stimulates the thyroid gland to distribute stored hormones.

4. TSH stimulates the secretion of T4 and T3.
   1. TSH secretion is regulated by T4 and T3 feedback inhibition.
   2. Low levels of T3 and T4  are stimuli for TSH and TRH.

5. Thyrotropin-releasing hormone (TRH) from the hypothalamus regulate TSH stimulation.
   1. TSH is the single most test for primary hypothyroidism.
6. In the damaged Pituitary and hypothalamus TSH and TRH are low or zero in spite of low T3 and T4.
7. If there is clear evidence of hypothyroidism and TSH is normal then think about the possibility of hypopituitarism.
   1. TSH level is high in primary hypothyroidism.
   2. TSH levels are low in the hyperthyroidism.

5. 3. Thyroid disorder = T4 decreased + TSH normal or raised
   4. Pituitary disorder = T4 decreased + TSH decreased

 

NORMAL

TSH

Source 1

Age	µU/mL
Premature infants 28 to 36 weeks gestation	0.7 to 27
1 to 4 days	1.0 to 39
2 to 20 weeks	1.7 to 9.1
5 months to 20 years	0.7 to 6.4
Adults
21 to 54 years	0.4 to 4.2
55 to 87 years	0.5 to 8.9
Pregnancy
First trimester	0.3 to 4.5
Second trimester	0.5 to 4.6
Third trimester	0.8 to 5.2

• 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)

• Adult = 0.4 to 4.2  µIU/L ( SI units are the same )
• Neonates = 3 to 20  µIU/L

Increased TSH level is seen in:

1. Adults and neonates with primary hypothyroidism.
   1. Congenital Cretinism.
2. Hashimoto’s thyroiditis.
3. The hypothyroid patient receiving inadequate treatment.
4. Thyrotoxicosis due to pituitary tumors.
5. Thyrotropin producing tumors e.g. ectopic TSH produced by the lung and breast cancers
6. TSH antibodies.
7. Large doses of iodine.
8. Chronic and severe illness.

Decreased TSH level is seen in:

1. Primary Hyperthyroidism.
2. Secondary and tertiary hypothyroidism.
3. Overtreatment in a hypothyroid patient.
4. Treated Grave’s disease patient.

Table showing values in Thyroid diseases

Clinical disease	TSH	T3	T4	Free T4
Primary Hypothyroidism	increased	decreased	deceased	deceased
Secondary Hypothyroidism	decreased	decreased	decreased	decreased
Primary Hyperthyroidism	decreased	increased	increased	increased
Secondary hyperthyroidism	increased	increased	increased	increased

Graves disease:

1. TSH is decreased.

2. Free T4 is increased

• Note please see more details on the thyroid function test.

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