Resistance to thyroid hormone is characterised by defects in the thyroid hormone receptors due to pathogenic genetic variants, resulting in reduced responsiveness to the action of thyroid hormone.

The X-linked FMR1 gene is associated with fragile X syndrome. Males with the condition have characteristic facial features and intellectual disability; female presentation varies and may include mood disorders. FMR1-related disorders in relatives include ataxia, tremor and early menopause.

At least 2.5% of patients with schizophrenia carry an identifiable neurodevelopmental copy number variant. This is likely to be more prevalent in patients with intellectual disability, neurodevelopmental conditions, congenital anomalies and/or a positive family history.

For some adults with epilepsy, there will be an underlying genetic cause.

Tuberous sclerosis complex (TSC) is a rare, variable, autosomal dominant genetic condition caused by pathogenic variants in the tumour suppressor genes TSC1 and TSC2. It is characterised by benign tumour growth and the variable presence of seizures and neurodevelopmental disorders.

PTEN hamartoma tumour syndrome, previously known as Cowden syndrome or Bannayan-Riley-Ruvalcaba syndrome, is an autosomal dominant genetic condition that leads to an increased risk of the patient developing benign and malignant tumours. The condition may be suspected in children with macrocephaly and developmental delay, specific dermatological features, vascular features (such as arteriovenous malformations or haemangiomas) and/or gastrointestinal polyps.

Developmental delay or intellectual disability is often multifactorial in nature and, in many cases, no specific cause is identified. For some children, particularly where the level of delay is moderate, severe or profound, there will be a genomic cause.

Overgrowth-intellectual disability (OGID) syndromes are a family of conditions associated with increased height and/or head circumference (at least two standard deviations above the mean (at the 98th centile)) and intellectual disability. Many different genetic causes of OGID syndromes have been identified.

Macrocephaly is defined as a head circumference over three standard deviations (3SD) above the mean for the patient’s age. In some affected children, there may be an underlying genomic cause.

Williams syndrome, caused by a microdeletion on chromosome 7 (position 7q11.23), is a developmental condition characterised by mild to moderate intellectual disability, distinctive facial features and cardiac defects (typically supravalvular aortic stenosis and peripheral pulmonary stenosis).