Thyroid stimulating hormone (TSH, thyrotropin) is formed in the cells of the anterior pituitary basophils and has a secretory sequence controlled by circadian TSH-releasing hormone (TRH) produced by the hypothalamus. The release of pituitary TSH is the central mechanism for regulating the biological action of thyroid hormones. TSH has a stimulatory action on the formation and secretion of thyroid hormones and proliferative action. Determination of TSH is the initial test in thyroid disease diagnosis. Even small changes in thyroid hormone concentrations will cause changes in the opposite direction (more pronounced) TSH concentrations. Therefore TSH is a highly specific and sensitive parameter for control of the thyroid function, and for the detection and exclusion of diseases of the hypothalamic-pituitary-thyroid axis.
TSH level is elevated in primary hypothyroidism (thyroid) being the most sensitive test for this condition. If there are clear signs of hypothyroidism, but the TSH level is not increased, secondary hypothyroidism is suspected in a hypopituitarism. Low levels are found in hyperthyroidism. TSH dosing is useful in monitoring the therapy in hypothyroid patients: low values are found in excessive doses of thyroid hormone replacement. Normal TSH values indicate a balanced treatment.
Recommendations for determination of TSH – thyroid disease diagnosis, differential diagnosis of disorders of the hypothalamic-pituitary-thyroid axis.
Patient preparation – fasting (fasting) 5; blood taken to determine thyroid TSH after puncture, recent biopsy, or after a surgical act including the thyroid.
There is a diurnal variation in the level of TSH: the maximum is 23 hours.
During pregnancy pregnant women show a slight downward trend in concentrations of TSH:
– First quarter: 0.33-4.59 μUI / mL;
– Second quarter: 0.35-4.10 μUI / mL;
– Third quarter: 0.21-3.15 μUI / mL.
Conversion factor: μUI / mL = mIU / L ..
Detection limit – 0.005 μUI / mL
Limits and interference
Approximately 3% of pregnant women show low or undetectable levels of TSH labeled due to increased concentrations of HCG. In secondary hypothyroidism (central) normal levels of TSH can be found, but its biological activity is low.