What bone age is

• Bones grow at the growth plates. At the start, the ends of the bone (epiphyses) are separate from the shaft (metaphysis).
• As a child matures, new ossification centres appear in a predictable order. At the end of puberty the epiphyses fuse with the metaphyses and growth stops.
• The appearance of these centres and the fusion of the plates show how "old" the skeleton is, whatever the child's actual age.
• It is read from a left hand and wrist X-ray. The reader compares it with a standard atlas (such as Greulich-Pyle) or with software.

What it tells you

1. How much of adult height has been reached.
2. How much growth is left. A child with a young bone age has more time to grow. A child with an advanced bone age has less.

The key rule

Why late diagnosis makes bone age lower than height age

Exam takeaway

• Short stature with a normal bone age suggests familial short stature.
• Short stature with a delayed bone age suggests constitutional delay, a chronic illness, an endocrine cause or undernutrition.
• Advanced bone age suggests precocious puberty.

The three ages

• Chronological age: the child's actual age, say 10 years.
• Height age: the age at which that height is average. If the child is as tall as a typical 7-year-old, the height age is 7.
• Bone age: the skeletal maturity on the hand X-ray.

What the passage says

Why this happens

• Height depends on how fast the growth plates add new bone length. Growth hormone, through IGF-1, drives this by stimulating growth plate cartilage cells to multiply.
• Bone maturation is the programmed sequence in which ossification centres appear and mature, and the growth plates eventually close. Thyroid hormone is a major driver of this process. Sex steroids drive it later in puberty. Growth hormone contributes too.
• Without thyroid hormone, or without adequate growth hormone, both processes slow down. They slow down at different rates.

1. The deficiency begins and growth slows. The child falls off the growth curve, so height age starts to lag.
2. Skeletal maturation also stalls, so bone age lags too.
3. Each year the deficiency goes untreated adds to both lags. In severe, long-standing hypothyroidism, ossification can be markedly delayed. In long-standing growth hormone deficiency, the bone age delay can also become substantial.
4. Bone age therefore drifts further behind, and it can end up below the height age.

Why it matters clinically

• A bone age lower than the height age points toward an endocrine cause such as hypothyroidism or growth hormone deficiency. It is not typical of simple constitutional delay or malnutrition, where bone age tracks height age.
• A very delayed bone age means a lot of growth potential is left. Once the hormone is replaced, the child can have catch-up growth, because the growth plates have stayed open.
• This is a pattern, not a diagnostic rule. Bone age alone does not establish the diagnosis. You confirm it with TSH and free T4 for hypothyroidism, and with IGF-1, IGFBP-3 and stimulation testing for growth hormone deficiency.

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