Mechanism of hypermelanosis in Addison disease

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Here is a complete, textbook-sourced explanation of the mechanism.

Mechanism of Hypermelanosis in Addison Disease

The Core Pathway: Loss of Negative Feedback

In primary adrenal insufficiency (Addison disease), the adrenal cortex is destroyed and fails to produce cortisol. Normally, cortisol exerts negative feedback on both the hypothalamus and the anterior pituitary to suppress secretion of CRH and ACTH respectively. When this feedback is lost, the pituitary corticotroph cells become hyperactive and massively upregulate transcription of the POMC gene.

"Persistently low cortisol levels do not provide negative regulatory feedback to the hypothalamus and pituitary. This results in unchecked production of POMC, ACTH, and melanocyte-stimulating hormone, which increase melanogenesis."

Fitzpatrick's Dermatology, p. 2543

POMC: The Key Precursor Molecule

Pro-opiomelanocortin (POMC) is a single large precursor polypeptide synthesised by the corticotroph cells of the anterior pituitary. Endopeptidases cleave it into several biologically active fragments simultaneously:

POMC cleavage diagram showing how POMC is split into ACTH intermediate + β-Lipotropin, then further into Fragment (containing γ-MSH), ACTH (containing α-MSH), γ-Lipotropin (containing β-MSH), and β-Endorphin

Fig. 9.10 - Hormones derived from POMC. Note that MSH activity is embedded in three separate fragments. (Costanzo Physiology, 7th ed., p. 414)

The critical point is that MSH sequences are embedded within the POMC molecule at multiple sites:

The fragment (leftover from ACTH intermediate cleavage) contains γ-MSH
ACTH itself contains α-MSH within its sequence
γ-Lipotropin contains β-MSH

Because POMC and ACTH are elevated together in Addison disease, all three MSH-containing fragments are simultaneously elevated.

How MSH Causes Hyperpigmentation

MSH (especially α-MSH) acts on MC1R (melanocortin-1 receptor) on dermal and epidermal melanocytes. This receptor activation:

Stimulates adenylyl cyclase → cAMP signalling
Upregulates tyrosinase activity (the rate-limiting enzyme in melanin synthesis)
Increases production and transfer of eumelanin (dark melanin) to surrounding keratinocytes

ACTH itself also has direct melanotrophic activity since it contains the α-MSH sequence, compounding the effect.

Why Hyperpigmentation is NOT Seen in Secondary/Tertiary Adrenal Insufficiency

This is a diagnostically important distinction:

Feature	Primary (Addison)	Secondary (pituitary)	Tertiary (hypothalamic)
ACTH level	High	Low	Low
POMC/MSH level	High	Low	Low
Hyperpigmentation	Present	Absent	Absent

In secondary and tertiary adrenal insufficiency, the pituitary either cannot produce ACTH (e.g., pituitary adenoma destroying corticotrophs) or lacks CRH stimulation. POMC and MSH levels are therefore low, and no pigmentary change occurs - Robbins, Cotran & Kumar, p. 1037.

Clinical Distribution of Hyperpigmentation

The pigmentation is generalised but accentuated in:

Sun-exposed areas (face, neck, dorsum of hands) - UV also stimulates local MSH release
Pressure points - elbows, knees, knuckles
Flexural folds and skin creases, including palmar creases
Scars (new scars formed during the disease show enhanced pigmentation)
Oral mucosa - buccal mucosa patches
Nails - longitudinal pigmented bands

Addison disease: hyperpigmentation of the face with accentuation in sun-exposed areas

Addison disease: hyperpigmented palmar creases (right) compared to a normal hand (left)

Clinical photos from Fitzpatrick's Dermatology, Fig. 137-12.

Summary Flowchart

Adrenal cortex destruction
        ↓
↓ Cortisol
        ↓
Loss of negative feedback on pituitary/hypothalamus
        ↓
↑ POMC transcription (corticotrophs)
        ↓
↑ POMC cleavage → ↑ ACTH + ↑ α-MSH + ↑ β-MSH + ↑ γ-MSH
        ↓
MSH binds MC1R on melanocytes
        ↓
↑ Tyrosinase → ↑ Melanin synthesis
        ↓
Generalised hyperpigmentation

Key point to remember: The hyperpigmentation is a marker of primary adrenal failure specifically - its presence implies the pituitary-hypothalamic axis is intact and responding normally to low cortisol. Its absence in secondary adrenocortical insufficiency is an equally useful clinical clue.

Sources: Costanzo Physiology 7th ed., p. 414; Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 1037; Fitzpatrick's Dermatology, p. 2543; Dermatology 2-Volume Set 5e.

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