Tyrosine hydroxylase deficiency

Tyrosine hydroxylase deficiency (THD) is a group of disorders caused by changing levels of the hormone dopamine. Dopamine sends signals between the nerve cells and the brain, to help control movements and emotional behaviour. Individuals with THD can vary in their symptoms and the age of onset of the condition.

Mild forms of the condition usually start in early childhood and include loss of muscle movement, difficulty walking and involuntary muscle tics. In more serious forms of the condition, babies are affected from birth and have poor muscle tone, muscle tremours, intellectual disability and brain disease.

Quick facts about tyrosine hydroxylase deficiency
Genes: TH
Inheritance: Autosomal Recessive
Alternative Names: THD
Relevant resources for tyrosine hydroxylase deficiency

A quick genetics rundown

As humans we have about 23,000 genes. These genes are like tiny instruction manuals that influence our health, growth and development. We inherit half of our genes from our biological mum and the other half from our biological dad. These genes are lined up on structures called chromosomes. Most of us have 23 pairs of chromosomes. The first 22 pairs are called autosomes and for the most part - these are the same among men and women. The 23rd pair determine our sex - two X chromosomes for a female and one X and one Y chromosome for males.

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How is tyrosine hydroxylase deficiency inherited?

tyrosine hydroxylase deficiency is known as an autosomal recessive condition. For autosomal recessive conditions, if a person has a variation in one copy of their gene, they are a carrier. This means that they are healthy because they also have a working copy of the gene. But, they can still pass their non-working copy to their child.

If the other parent also happens to be a carrier of the same gene, there is a 25% (1 in 4) chance that they both pass this gene variation on to their child — and as such, have a child affected by the disease.

If both parents are carriers of tyrosine hydroxylase deficiency, there’s a one in four chance that their children could develop symptoms.
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What is carrier screening?

Carrier testing is like a checkup for your genes. It tests to see if you carry a gene variation that could cause a serious genetic disease in your child. Eugene offers an inclusive genetic carrier screening panel that includes tyrosine hydroxylase deficiency, but there's a total 301 conditions that can be tested.

Eugene’s carrier test is a clinical grade test that can be done from the comfort of your own home — it’s just a saliva test. You're also paired with a genetic counsellor who provides mindful support and guidance every step of the way.

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Should parents screen for tyrosine hydroxylase deficiency before or early in pregnancy?

The biggest benefit of screening for tyrosine hydroxylase deficiency is that it can help future parents understand their reproductive risk so they can be ready and empowered to make more informed decisions. If neither partner are carriers, it provides reassurance and peace of mind that the risk of having a child with a genetic disease is low.

Since 90% of children that have a recessive genetic disease like tyrosine hydroxylase deficiency had no previous family history of it, it often feels completely out of the blue for the parents. Getting screened is a way to know this risk in advance, which can help familes manage or even prevent the disease in the first place.

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