IGF-1

Growth hormone is released from your pituitary in short pulses, the largest in the first hours of your deep sleep. A single measurement of growth hormone therefore says little: you catch a peak or a trough. The hormone sets your liver to making IGF-1, insulin-like growth factor 1, and that does the actual work: it makes cells grow and divide, builds muscle and bone and maintains tissue. IGF-1 sits stably in your blood for days, and that makes it the practical measure of how much growth hormone your body makes. The number moves with your life. IGF-1 is high during growth and peaks in puberty, then falls steadily and at seventy is roughly half of what it was at thirty. It drops when you eat too little, with too little protein and with a liver that makes less, and it rises with a pituitary that releases too much growth hormone or with growth hormone from outside. Your report shows IGF-1 in nmol/L, but because the normal range shifts with your age, no fixed band comes with it. Instead your value is converted into an SD score: the number of standard deviations you sit from the mean of your own age and sex. Between -2.00 and +2.00 SD is normal, and 0 is exactly the mean of your group.

IGF-1 is a value that says something about maintenance. In adulthood growth hormone and IGF-1 keep your muscle mass, your bone density and your recovery up, and they fall with the years. An IGF-1 at the low end of your age group goes together with less muscle, more belly fat and slower recovery; it is one of the numbers that shows whether your body is still in build-up mode. The levers are known: deep sleep, because that is where most growth hormone is released, strength training, enough protein, and not chronically eating too little. That last one is the most common reason for a low IGF-1 in a healthy person. When you eat too little, on a strict diet or with too little protein, the liver turns production down, even when there is enough growth hormone; the body does not build when nothing comes in. The same holds for an underactive thyroid, poorly controlled diabetes and a liver that works less. A low IGF-1 with a normal pattern of eating, sleeping and training is the exception, and then the pituitary comes into view. At the high end the story is different. An IGF-1 far above your age group without you using growth hormone points at a pituitary releasing too much, acromegaly, with larger hands and feet, a coarser face, sweating and joint complaints that come on so gradually they are missed for years. IGF-1 is the first test there. And anyone using growth hormone as doping or an anti-ageing agent lifts the number, with in the long run more insulin resistance and a higher risk of some cancers: a high IGF-1 is not a goal.

Read the SD score first, not the concentration. It sets your number against your own age and sex, and without that scale the concentration in nmol/L cannot be placed; if it is not there, ask for it. A low SD score means less growth hormone action than usual for your age. The most common explanation first: eating too little, a strict diet or too little protein, an underactive thyroid, a liver that works less or poorly controlled diabetes, little deep sleep, and rarely a pituitary that makes too little growth hormone. In a healthy person who diets or eats little, the value recovers within weeks of eating normally. A high SD score means more growth hormone action. In pregnancy and in late puberty that is as it should be. Beyond that it is almost always growth hormone from outside, or rarely a pituitary releasing too much; the latter deserves investigation, because acromegaly is very treatable and harmful if it runs on for years.