Growth hormone

GH testing is usually provocative using either a GH stimulation test or a GH suppression test to track GH levels over time. GH testing may be used to test for abnormal pituitary function and to help diagnose the condition causing the abnormality, its severity, and the complications that have arisen because of it.

Often other blood tests that reflect pituitary function, such as T4, TSH, cortisol, FSH, LH, and testosterone (in men), are also ordered. These tests are usually performed prior to GH testing to make sure that they are normal and/or controlled with medication before GH testing is done. Glucose levels are run on the samples collected during the GH suppression test, both to track glucose levels and to make sure that the patient’s system is sufficiently challenged by the glucose solution.

IGF-1 (Insulin-like growth factor–1) is often measured once during GH provocation testing and is often used by itself or with GH as a monitoring tool. Produced in the liver, IGF-1 mirrors GH excesses and deficiencies, but its level is stable throughout the day, making it a useful indicator of average GH levels.

GH testing is usually ordered on those with symptoms of growth hormone abnormalities or as a follow-up to other abnormal hormone test results. It is not recommended for general screening. GH tests may be ordered to help evaluate pituitary function:

• GH stimulation tests are used to test for hypopituitarism
• GH suppression tests are used to test for hyperpituitarism

Periodic GH measurements may also be performed on children who have had radiation treatment of the central nervous system or whole body irradiation prior to stem cell transplants. This is common in acute lymphoblastic leukaemia (ALL), where irradiation of the brain can affect the hypothalamus and pituitary gland, thus affecting growth.

Growth hormone tests help identify excess and diminished GH production and give your doctor information about the severity of your condition and are part of the diagnostic work-up to find the reason for abnormal hormone production.

• GH stimulation tests help diagnose growth hormone deficiency (GHD) in children and adults.
• GH suppression tests help diagnose gigantism in children and acromegaly in adults. Along with other blood tests and imaging scans, they help identify and locate pituitary tumours

GH and IGF-1 levels are often monitored for extended periods of time following treatment for GH deficiency, gigantism and acromegaly, and are monitored following surgery, drug treatment, and/or radiation therapy for a pituitary tumour.

GH stimulation testing is ordered when your child has symptoms of growth hormone deficiency (GHD), such as when:

• your child's growth rate slows down in early childhood and he or she is significantly shorter than others of the same age
• TSH tests show that your child is not hypothyroid (low thyroid levels can also cause slowed growth)
• X-rays show delayed bone development
• your doctor suspects that your child’s pituitary gland is under-active.

Once GHD is diagnosed, your doctor may use stimulation testing to confirm the diagnosis, along with IGF-1 to monitor the effectiveness of GH replacement (if indicated), and as a child reaches adulthood to see if continued supplementation is necessary. GH levels are also monitored in children who have received radiation therapy.

Stimulation testing is ordered in adults when patients have symptoms of GHD and/or hypopituitarism, such as decreased bone density, fatigue, adverse lipid changes, and reduced exercise tolerance. Other hormone testing is done first to rule out other conditions that may cause similar symptoms.

GH suppression testing is done when children show signs of gigantism, when adults show signs of acromegaly, and/or when their doctor suspects hyperpituitarism. Suppression testing may be done when a pituitary tumour is suspected and may be used along with IGF-1 levels and other hormone levels to monitor the effectiveness of treatment for these conditions. Monitoring may continue at regular intervals for many years to watch for recurrence.

Since GH is released by the pituitary in bursts, random GH levels are not very useful. There is too much overlap between abnormal results and normal daily variation. GH levels will be higher first thing in the morning and will increase with exercise and stress.

GH suppression test:
If your GH levels are not significantly suppressed during a GH suppression test (they stay higher than they should) and:

• you have symptoms of gigantism or acromegaly
• other pituitary hormone levels are normal and/or controlled
• your IGF-1 levels are high

then it is likely that you are producing too much GH and it is causing complications. If you have other pituitary hormones that are abnormal, then you may have a condition causing hyperpituitarism. If a mass shows up on an X-ray, CT scan, or MRI scan, then you may have a pituitary tumour (or very rarely a malignancy). If you are being monitored for a previous tumour, then you may be having a recurrence.

GH stimulation test:
If your GH levels are not significantly stimulated during a GH stimulation test (they stay lower than they should) and:

• you have symptoms of GHD
• other pituitary hormone levels are normal and/or controlled
• your IGF-1 level is low

then it is likely that you have a deficiency of GH that your doctor may treat. If your TSH level is low, then that should be addressed first as thyroid deficiencies can cause symptoms similar to GHD. You may also have a more general decrease in pituitary function.

Pituitary tumours are the most common cause of excess GH production but may also cause deficiencies. If the tumour produces another of the pituitary hormones, such as ACTH or prolactin, it may inhibit GH secretion. If the tumour is relatively large, it may inhibit all pituitary hormone production and cause damage to the surrounding tissues. Some other conditions also cause general hyper- or hypopituitarism. These may be genetic, due to disease, or as a result of trauma.

Factors that can interfere with GH testing include:

• Stress, exercise, and low blood glucose levels
• Drugs that can increase GH include: amphetamines, arginine, dopamine, oestrogens, glucagon, histamine, insulin, levodopa, methyldopa and nicotinic acid
• Drugs that can decrease GH levels: corticosteroids and phenothiazines
• Radioactive scan within week of test (with some laboratory methods)

Abnormal GH levels can usually be modified once the causes are identified. Synthetic GH is available to alleviate deficiencies in children (treatment of adults with GHD is more controversial). Combinations of surgery, medication and radiation can be used to treat pituitary tumours that are causing excess GH production. The important thing is to identify GH abnormalities as soon as possible for good outcomes. The bone growth changes associated with gigantism and acromegaly are permanent and, if left untreated, the GH deficient child’s short stature will remain.

There can be long term complications from GH abnormalities. Acromegaly, for instance, can cause colon polyps (increasing a patient’s risk of developing colon cancer), diabetes, high blood pressure, and visual abnormalities. If a pituitary tumour permanently damages pituitary cells, then multiple hormone replacement may be necessary. Increased bone growth may also lead to trapped nerves (carpal tunnel syndrome), arthritis, and weak bones.