Thyroid Stimulating Hormone (TSH)

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Latest Edit: Hector 2014-3-21 (EDT)

See Also Lab Tests

A negative feed-back control system is responsible for thyroid hormone synthesis and secretion. Thyrotropin-releasing hormone or TRH is secreted by the hypothalamus and stimulates the anterior pituitary to secrete TSH. TSH or thyroid stimulating hormone then acts on the thyroid gland to stimulate the release of T3 and T4, which act negatively on the anterior pituitary gland to suppress the release of TSH, thereby controlling thyroid hormone production.[1],[2]


Contents

Discussion

  • TSH is the most sensitive test for primary hypothyroidism
  • A decreased T3 and T4 causes an increase in TSH, and an increase in T3 and T4 causes a decrease, with normal thyroid and anterior pituitary/hypothalamus function
  • TSH is useful for the determination and potential differentiation of hypothyroidism
  • Decreased TSH levels may indicate a hyperthyroid state
  • TSH is slow to respond to thyroid hormone supplementation. It is advised to wait a minimum of 4 to 6 weeks before retesting.[3]

Patient Preparation

  • No food or drink restrictions are necessary

A number of factors can interfer with TSH results.[3]

  • Factors which can cause increased levels:
  • Drugs: Administration of radioisotopes within one week prior to testing, antithyroid medications, lithium, potassium iodide, amiodarone and TSH injection
  • Factors which can cause decreased levels: severe illness
  • Drugs: Aspirin, dopamine, heparin, steroids, and l-thyroxine, triiodothyronine, amiodarone and iodine.

Clinical Implications

Ranges: The following are the reference ranges for this lab. However, lab ranges can vary by laboratory and country. [2]

Standard U.S. Units Standard International Units
Conventional Laboratory Range 0.35-5.5 uIU/ml 0.35-5.5 mIU/L
Optimal Range 2.0-4.4 uIU/ml 2.0-4.4 mIU/L
Alarm Ranges < 0.3 or > 10.0 uIU/ml < 0.3 or > 10.0 mIU/L

High levels indicate:

Low levels indicate:

  • Hyperthyroidism
  • Multinodular goiter
  • Secondary hypothyroidism (anterior pituitary hypofunction)
  • Tertiary hypothyroidism (hypothalamus hypofunction)
  • Heavy metal body burder (e.g. lead, aluminum, cadmium, and other toxic metals)
  • Other conditions: protein malnutrition, Hashimoto's thyroiditis, pregnancy, sub-acute thyroiditis.

Associated Tests

References

  1. Pagana Kathleen D, Pagana Timothy J (1998) Mosby's Manual of Diagnostic and Laboratory Tests, Mosby, Inc
  2. 2.0 2.1 Weatherby Dicken, Ferguson Scott (2002) Blood Chemistry and CBC Analysis: Clinical Laboratory Testing from a Functional Perspective, Bear Mountain
  3. 3.0 3.1 Guidelines for the Use of Laboratory Tests to Detect Thyroid Dysfunction (2007) Ontario Association of Medical Laboratories.
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