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Estradiol disrupts thyroid hormone production because high blood levels of estrogen signal the liver to increase the production of thyroid-binding globulin (TBG). This is an inhibitor protein that binds to the thyroid hormone, reducing the amount of T 3 and T 4 available for use by cells. [37]
Thyroid hormone uptake (T uptake or T 3 uptake) is a measure of the unbound thyroxine binding globulins in the blood, that is, the TBG that is unsaturated with thyroid hormone. [2] Unsaturated TBG increases with decreased levels of thyroid hormones. It is not directly related to triiodothyronine, despite the name T 3 uptake. [2] Reference ranges:
The TSH, in turn, stimulates the thyroid to produce thyroid hormone until levels in the blood return to normal. Thyroid hormone exerts negative feedback control over the hypothalamus as well as anterior pituitary, thus controlling the release of both TRH from hypothalamus and TSH from anterior pituitary gland.
The test is used in the differential diagnosis of secondary and tertiary hypothyroidism. First, blood is drawn and a baseline TSH level is measured. Then, TRH is administered via a vein. After 30 minutes blood is drawn again and the levels of TSH are measured and compared to the baseline. Some authors recommend additional blood sampling at 15 ...
However, of children who develop anti-thyroid antibodies and hypothyroidism, up to 50% are later observed to have normal antibodies and thyroid hormone levels. [ 5 ] One case of true remission has been observed in a 12-year-old girl.
Still, the blood values are approximately equal between the arterial and venous sides for most substances, with the exception of acid–base, blood gases and drugs (used in therapeutic drug monitoring (TDM) assays). [6] Arterial levels for drugs are generally higher than venous levels because of extraction while passing through tissues. [6]
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