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
- 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.
Pathophysiology
- 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 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 the hyperthyroidism.
-
- Thyroid disorder = T4 decreased + TSH normal or raised
- 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:
- Adults 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 |
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.