Thyroid:- Part 2 – TSH (Thyroid-Stimulating Hormone), Thyrotropin
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)
- TSH differentiates Hypothyroidism from hyperthyroidism.
- TSH is used to diagnose primary hypothyroidism.
- TSH is used to differentiate primary from secondary hypothyroidism.
- TSH also differentiates hypothyroidism due to the hypothalamus (Tertiary hypothyroidism).
- TSH level is also done for suppressive or replacement therapy.
- TSH has also been done in a newborn with primary hypothyroidism with a low T4 level.
Definition of thyroid-stimulating hormone (TSH):
- TSH is produced by the anterior pituitary gland.
- TSH plays an important role in regulating the thyroid glands’ activity, which produces important hormones for the body.
- TSH controls the metabolism, growth, and development of the body.
Structure of Thyroid-Stimulating Hormone (TSH)
- TSH is a glycoprotein hormone composed of two polypeptide chains, alpha, and beta.
- Alpha (α) subunit consists of 92 amino acids.
- The beta (β) subunit consists of 118 amino acids, and the biological activity is because of the Beta subunit.
- TSH molecular weight is 28,000 Da.
- It arises from the prehormone and prohormone.
- It has a short half-life of minutes to a few hours compared to other hormones like steroids.
- TSH binds with the specific receptors for TSH found in the cytoplasmic membrane of the thyroid cells.
- It releases the second messenger that activates cAMP and protein kinase and stimulates further biochemical reactions.
- TSH is transported free and not bound to protein carriers in the body fluids.
- With other adrenergic neuropeptides, TSH binds to the cytoplasm membrane receptors, and the thyroid gland secretes the T4 and T3.
- The thyroid gland has large hormone stores and a slow normal turnover rate.
Role of TSH:
- The pituitary gland produces TSH, which stimulates the thyroid gland to distribute stored hormones.
- TSH stimulates the secretion of T4 and T3.
- TSH secretion is regulated by T4 and T3 feedback inhibition.
- Low levels of T3 and T4 are stimuli for TSH and TRH.
- TSH influences our bodies in different ways.
TSH and thyroid gland functions:
- TSH stimulates the thyroid gland and leads to the secretion of T3 and T4.
- T3 and T4 have all these specific functions.
- Thyrotropin-releasing hormone (TRH) from the hypothalamus regulates TSH stimulation.
- TSH is the single most test for primary hypothyroidism.
- In the damaged Pituitary and hypothalamus, TSH and TRH are low or zero in spite of low T3 and T4.
- If there is clear evidence of hypothyroidism and TSH is normal, then think about the possibility of hypopituitarism.
- TSH level is high in primary hypothyroidism.
- TSH levels are low in hyperthyroidism.
- Thyroid disorder = T4 decreased + TSH normal or raised
- Pituitary disorder = T4 decreased + TSH decreased
NORMAL TSH level
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|
|21 to 54 years||0.4 to 4.2|
|55 to 87 years||0.5 to 8.9|
|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
- 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.
- 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)
- 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:
- Adults and neonates with primary hypothyroidism.
- Congenital Cretinism.
- Hashimoto’s thyroiditis.
- The hypothyroid patient received inadequate treatment.
- Thyrotoxicosis due to pituitary tumors.
- Thyrotropin-producing tumors, e.g., ectopic TSH produced by lung and breast cancers
- TSH antibodies.
- Large doses of iodine.
- Chronic and severe illness.
Decreased TSH level is seen in:
- Primary Hyperthyroidism.
- Secondary and tertiary hypothyroidism.
- Overtreatment in a hypothyroid patient.
- Treated Grave’s disease patient.
Thyroid hormones in various diseases
|Clinical disease||TSH||T3||T4||Free T4|
1. TSH is decreased.
2. Free T4 is increased
Questions and answers:
Question 1: What will be the value of TSH in hypothyroidism?
TSH value is increased in hypothyroidism.
Question 2: What will be the value of TSH in hyperthyroidism?
TSH value will be very low in hyperthyroidism.
- Note, please see more details on the thyroid function test.