THYROID STIMULATING HORMONE

Thyroid Stimulating Hormone

The thyroid is a unique organ of the endocrine glands because it has a large store of hormone and a slow rate of normal turnover. Stimulation of the thyroid gland by the TSH, which is produced by the anterior pituitary gland, causes the release and distribution of stored thyroid hormones. TSH stimulate secretion of T4 and T3. TSH secretion is physiolologically regulated by T3 and T4 (feedback inhibition) and is stimulated by thyrothropin-releasing hormone (TRH) from the hypothalamus. TSH is the single most sensitive test for primary hypothyroididsm. If there is clear evidence of hypothyroidism and the TSH is not elevated, then an implication of possible hypopituitarism.

This test is used in the diagnosis of primary hypothyroidism when there is thyroid gland failure owing to intrinsic disease, and it is used to differentiate primary from secondary hypothyroidism by determining the circulatory level of TSH. High TSH levels are seen in primary hypothyroidism and low TSH levels are seen in hyperthyroidism.

TSH measurements will distinguish low levels from normal levels which have become the preferred test for hyperthyroidism. The high-sensitivity TSH test is useful for diagnosing sick euthyroid patients and in differentiating mild hyperthyroidism from Grave’s disease.

Reference Values:

Adults: 0.4-4.2 mIU/L
Neonates: 3-20 µIU/L by day 3 of life

Clinical Implications

1. ↑ TSH levels are seen in the following conditions:
a. Adults and Neonates with primary hypothyroidism
b. Thyrothropin-producing tumor (ei. ectopic TSH secretion from lung, breast tumors)
c. Hashimoto’s thyroiditis
d. Thyrotoxicosis due to pituitary tumor
e. TSH antibodies
f. Hypothyroid patients receiving insufficient thyroid replacement hormone or thyroid hormone resistance

2. ↓TSH levels are associated with the following conditions:
a. Primary hyperthyroidism
b. Secondary and tertiary hypothyroidism
c. Treated Grave’s disease
d. Euthyroid sick disease
e. Overreplacement of thyroid hormone in treatment of hypothyroidism

Interferring Factors
• Values are normally high in neonatal cord blood because of hypersecretion of TSH in newborns. This level will be normal after 1st week of life.
• Treatment with thyroxine and corticosteroids will decrease level of TSH.
• Iodine containing drugs will increase level of TSH in the body.
• TSH values are decreased in the first trimester of pregnancy.
• TSH values are increased in older people (>80 yrs. old)

TSH subunits

TSH consists of two subunits, the alpha and the beta subunit.
- The alpha subunit is identical to that of human chorionic gonadotropin (HCG), luteinizing hormone (LH) and follicle stimulating hormone (FSH).
- The beta subunit is unique to the TSH.

The TSH receptor

The TSH receptor is found mainly on thyroid follicular cells. Stimulation of the receptor increases the T3 and T4 secretion and production.

Therapeutic Use

A drug which is a recombinant human TSH, called Thyrogen is used in patients with thyroid cancer.

References:
• Fischbach, Frances, A manual of Laboratory and Diagnostic test, 7th edition

• http://en.wikipedia.org/wiki/Thyroid-stimulating_hormone