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The primary test to measure luteotropic hormone (LTH), or prolactin, is simply called prolactin (PRL). Related prolactin tests such as follicle-stimulating hormone (FSH) and luteninizing hormone (LH) help to evaluate potential causes of abnormal LTH levels. Along with the aforementioned tests, total testosterone and thyroid-stimulating hormone (TSH) are also measured to help diagnose pituitary hormone disorders.
LTH is a type of protein that is encoded by the human PRL gene, which is primarily associated with lactation in pregnant mothers. Both men and women produce varying amounts of this protein. In women, LTH amounts will typically increase during pregnancy and breast-feeding. Typical LTH levels for non-pregnant women are less than 25 micrograms per liter (1,086.95 pmol/L), while typical LTH levels in men are less than 20 micrograms per liter (869.56 pmol/L). The level of LTH in men will remain steady on a daily basis, but levels will gradually decrease with age.
Doctors will typically order prolactin tests if hormonal dysfunction is apparent. For example, women suffering from the absence of menses, infertility and galactorrhea — which is lactation without pregnancy — will likely warrant a PRL and related tests. Symptoms of hormonal issues in men include erectile dysfunction, loss of libido, low testosterone and galactorrhea. With the exception of pregnant women, a PRL test with an LTH level over 250 micrograms per liter (10,869.5 pmol/L) is considered highly abnormal and may signal a pituitary tumor.
When elevated or abnormal PRL results along with the associated symptoms are indicated, additional tests such as FSH and LH will most likely be performed. Both FSH and LH tests are used to measure the amount of hormone that is synthesized and secreted by the pituitary gland. FSH performs different functions in men and women. In men, FSH helps to control the production of sperm, while the hormone helps to control a woman’s menstrual cycle. Normal FSH test ranges for pre-menopausal women vary between 2 to 16 international units per liter (IU/L), while 1 to 8 IU/L is normal for men.
Similar to FSH, LH is responsible for stimulating the production of important reproductive functions in both genders. LH levels between 1 to 17 international units per liter in women will trigger normal ovulation levels, while levels between 1 to 15 IU/L will stimulate the normal production of testosterone for men. When LTH levels are elevated, FSH and LH test results will most likely be in the lower range.
Another pair of related prolactin tests is called total testosterone and TSH. The total testosterone test measures the amount of free and bound testosterone in the blood. Free testosterone is the amount of testosterone not bound to any other chemical in the body. Bound testosterone refers to the amount of testosterone that is loosely bound to a protein called albumin.
Sometimes the total testosterone test is a precursor to the PRL test, especially when sexual or reproductive dysfunction is present in men. Testosterone is unusually low when LTH levels are high. Depending on age, normal testosterone levels in adult men vary between 200 to 1,080.12 nanograms per deciliter (6.94 to 37.48 nmol/L). Women also produce testosterone at much lower concentrations of between 8.07 to 70.03 nanograms per deciliter (0.28 to 2.43 nmol/L).
The TSH test can be used as a diagnostic tool to measure thyroid function when elevated LTH levels are present. A thyroid issue such as hypothyroidism is usually present with TSH levels higher than 4.5 milli-international units per liter. Elevated levels on both TSH and prolactin tests may point to hypothyroidism, which is the inability of the thyroid grand to make enough hormones. Those who suffer from hypothyroidism often experience weight gain, fatigue and sensitivity to cold. Hypothyroidism coupled with high LTH levels may be caused by a pituitary tumor.