Blood tests to measure hormone levels

An evaluation usually begins by measuring blood levels of the hormones that the pituitary gland produces (typically, thyroid-stimulating hormone, prolactin, luteinizing hormone, and follicle-stimulating hormone) and at the same time measuring levels of the hormone produced by the target organs (typically, thyroid hormone, testosterone in men, and estrogen in women).

For example, a person with hypothyroidism due to failure of the pituitary gland has low levels of thyroid hormone and low or inappropriately normal levels of thyroid-stimulating hormone, which is produced by the pituitary gland. In contrast, a person with hypothyroidism due to failure of the thyroid gland itself has low levels of thyroid hormone and high levels of thyroid-stimulating hormone.

Growth hormone production by the pituitary is difficult to evaluate because no single blood level accurately reflects it. The body usually produces growth hormone in several bursts a night, and the hormone is quickly used. Thus, the blood level at any given moment does not indicate whether production is normal over the course of a day. Instead, doctors measure the levels of insulin-like growth factor 1 (IGF-1) in the blood. Production of IGF-1 is controlled by growth hormone, and the level of IGF-1 tends to change slowly in proportion to the overall amount of growth hormone produced by the pituitary. In infants and young children, doctors may instead measure levels of a similar substance, IGF-binding protein type 3. Measurement of IGF-1 is not sufficient to make the diagnosis of growth hormone deficiency in adults, because people with normal levels may still have growth hormone deficiency. In many cases, a stimulation test to try to make the pituitary secrete growth hormone is used.

Because the levels of luteinizing hormone and follicle-stimulating hormone fluctuate with the menstrual cycle, their measurement in women may be difficult to interpret. However, in postmenopausal women who are not taking estrogen, luteinizing hormone and follicle-stimulating hormone levels normally are high. When they are found to be low, this can be an indication of pituitary damage or insufficiency of other hormones.

Production of ACTH is usually assessed by measuring the levels of its target hormone (cortisol) in response to stimuli, such as an injection of synthetic ACTH (ACTH stimulation test) or a low level of sugar in the blood after an insulin injection (insulin tolerance test). If the level of cortisol does not change and the level of ACTH in the blood is normal or low, a deficiency of ACTH production is confirmed.

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Hypopituitarism - Hormonal and Metabolic Disorders - Merck Manuals ...

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