Tyrosine !

Tyrosine comes from phenylalanine and is the starting point of dopamine, adrenaline, melanin and thyroid hormones.

Updated 2026-11-01

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Key facts

Non-essential while phenylalanine hydroxylase works; essential in phenylketonuria.
Tyrosine hydroxylase → L-DOPA → dopamine → noradrenaline → adrenaline.
Tyrosinase turns it into melanin in melanocytes.
In the thyroid, tyrosine residues of thyroglobulin are iodinated to form T3 and T4.
It is broken down to fumarate and acetoacetate.

Like phenylalanine with a hydroxyl on the ring (a phenol); in proteins that OH can be phosphorylated.

Albinism (tyrosinase), alkaptonuria and the tyrosinaemias are defects of its metabolism.
Phaeochromocytoma and neuroblastoma make excess catecholamines from it.
In phenylketonuria tyrosine is short: lighter skin and hair from less melanin.

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What is the rate-limiting step of catecholamine synthesis from tyrosine?

Tyrosine hydroxylase, which makes L-DOPA with BH4, oxygen and iron.

Why do people with phenylketonuria often have light skin and hair?

They have little tyrosine, and high phenylalanine inhibits tyrosinase, so less melanin is made.

Which hormones are made from tyrosine residues in the thyroid?

T3 and T4, when tyrosine residues of thyroglobulin are iodinated and coupled.

Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: L-Tyrosine (CID 6057) (PubChem, NCBI, 2026) · Public domain; Tyrosine catabolism (Reactome, 2026); Catecholamine biosynthesis (Reactome, 2026).

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