Thyroid – Part 2 – Thyroid stimulating hormone (TSH), Thyrotropin
- Venous blood is taken and the test is done on the serum of the patient.
- TSH differentiates Hypothyroidism from hyperthyroidism.
- TSH used to diagnose primary hypothyroidism.
- TSH used to differentiate primary from secondary hypothyroidism.
- TSH also differentiates hypothyroidism due to the hypothalamus (Tertiary hypothyroidism).
- TSH level is also done for the suppressive or replacement therapy.
- TSH also has been done in a newborn with primary hypothyroidism who has a low T4 level.
- TSH is a glycoprotein hormone composed of two polypeptide chains alpha and beta.
- It arises from the prehormone and prohormone.
- It has a short half-life of minutes to a few hours as 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 carrier in the body fluids.
- TSH with other adrenergic neuropeptides binds to the cytoplasm membrane receptors and the thyroid gland secretes hormone T4 and T3.
- The thyroid gland has large stores of hormones and has a slow rate of normal turnover.
- 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.
- Thyrotropin-releasing hormone (TRH) from the hypothalamus regulate TSH stimulation.
- TSH is the single most test for primary hypothyroidism.
- In 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 the hyperthyroidism.
- Thyroid disorder = T4 decreased + TSH normal or raised
- Pituitary disorder = T4 decreased + TSH decreased
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:
- Adult and neonates with primary hypothyroidism.
- Congenital Cretinism.
- Hashimoto’s thyroiditis.
- The hypothyroid patient receiving inadequate treatment.
- Thyrotoxicosis due to pituitary tumors.
- Thyrotropin producing tumors e.g. ectopic TSH produced by the 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.
Table showing values in Thyroid diseases
|Clinical disease||TSH||T3||T4||Free T4|
1. TSH is decreased.
2. Free T4 is increased
- Note please see more details on thyroid function test