TSH (Thyroid-Stimulating Hormone)

Sample for Thyroid stimulating hormone

• Venous blood is taken, and the test is done on the patient’s serum.

Indications for Thyroid-Stimulating Hormone (TSH)

1. TSH differentiates Hypothyroidism from hyperthyroidism.
2. TSH is used to diagnose primary hypothyroidism.
3. TSH is 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 suppressive or replacement therapy.
6. TSH has also been done in a newborn with primary hypothyroidism with a low T4 level.

Definition of thyroid-stimulating hormone (TSH):

1. TSH is produced by the anterior pituitary gland.
2. TSH plays an important role in regulating the thyroid glands’ activity, which produces important hormones for the body.
3. TSH controls the metabolism, growth, and development of the body.

Structure of Thyroid-Stimulating Hormone (TSH)

1. TSH is a glycoprotein hormone composed of two polypeptide chains, alpha, and beta.
   1. Alpha (α) subunit consists of 92 amino acids.
   2. The beta (β) subunit consists of 118 amino acids, and the biological activity is because of the Beta subunit.
   3. TSH molecular weight is 28,000 Da.

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

Role of TSH:

1. The pituitary gland produces TSH, which stimulates the thyroid gland to distribute stored hormones.
2. 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.

TSH interpretations:

1. Thyrotropin-releasing hormone (TRH) from the hypothalamus regulates TSH stimulation.
   1. TSH is the single most test for primary hypothyroidism.
2. In the damaged Pituitary and hypothalamus, TSH and TRH are low or zero in spite of low T3 and T4.
3. 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 hyperthyroidism.

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

NORMAL TSH level

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)

• 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 the following:

1. Adults and neonates with primary hypothyroidism.
   1. Congenital Cretinism.
2. Hashimoto’s thyroiditis.
3. The hypothyroid patient received inadequate treatment.
4. Thyrotoxicosis due to pituitary tumors.
5. Thyrotropin-producing tumors, e.g., ectopic TSH produced by 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.

Thyroid hormones in various diseases

Graves disease:

1. TSH is decreased.

2. Free T4 is increased

Questions and answers:

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