The level of thyroid hormones (T3 and T4) in the blood have an additional effect on the pituitary release of TSH; When the levels of T3 and T4 are low, the production of TSH is increased, and conversely, when levels of T3 and T4 are high, then TSH production is decreased. This effect creates a regulatory negative feedback loop.
Stimulating antibodies to this receptor mimic TSH action and are found in Graves' disease.
TSH levels for children normally start out much higher. In 2002, the National Academy of Clinical Biochemistry (NACB) in the United States recommended age-related reference limits starting from about 1.3-19 uIU/mL for normal term infants at birth, dropping to 0.6-10 uIU/mL at 10 weeks old, 0.4-7.0 uIU/mL at 14 months and gradually dropping during childhood and puberty to adult levels, 0.4-4.0 uIU/mL.
The NACB also stated that it expected the normal (95%) range for adults to be reduced to 0.4-2.5 uIU/mL, because research had shown that adults with an initially measured TSH level of over 2.0 uIU/mL had "an increased odds ratio of developing hypothyroidism over the [following] 20 years, especially if thyroid antibodies were elevated".
|Source of pathology||TSH level||thyroid hormone level||Disease causing conditions|
|hypothalamus/pituitary||high||high||benign tumor of the pituitary (adenoma) or thyroid hormone resistance|
|thyroid||low||high||hyperthyroidism or Grave's disease|
|thyroid||high||low||congenital hypothyroidism (cretinism), hypothyroidism|
Clearly, both TSH and T3 and T4 should be measured to ascertain where a specific thyroid dysfunction is caused by primary pituitary or by a primary thyroid disease. If both are up (or down) then the problem is probably in the pituitary. If the one component (TSH) is up, and the other (T3 and T4) is down, then the disease is probably in the thyroid itself. The same holds for a low TSH, high T3 and T4 finding.
A TSH assay is now also the recommended screening tool for thyroid disease. Recent advances in increasing the sensitivity of the TSH assay make it a better screening tool than free T4.