Peptides

Adrenocorticotrophic hormone (ACTH) is a 39 amino acid peptide hormone secreted by the anterior pituitary, under the control of the hypothalamic peptide, corticotrophin-releasing hormone (CRH). ACTH secretion is pulsatile and exhibits diurnal variation, with highest plasma concentrations around 8am and lowest levels at midnight. It stimulates glucocorticoid (cortisol) production in the adrenal cortex.

ACTH measurement is only useful as a second line investigation following the finding of either cortisol deficiency or excess.

In cortisol deficiency due to primary adrenal failure, ACTH will be raised due to lack of negative feedback. ACTH will be low in adrenal insufficiency secondary to pituitary failure (hypopituitarism).

Excessive production of cortisol accompanied by suppressed ACTH may be seen in Cushing’s syndrome due to adrenal tumours/hyperplasia, and with exogenous glucocorticoid administration.
Cortisol excess with raised ACTH may be seen in ACTH-producing tumours of the pituitary (Cushing’s disease) or other tissues e.g. lungs (ectopic ACTH production).

NB. ACTH secretion may be increased by stress.

Sample Requirements and Reference Ranges

• Sample Type: Plasma
• Container: EDTA
• Precautions: Separate and freeze plasma within 4 hours of sample collection. Transport frozen. Timing of collection important. Avoid stress. Haemolysed specimen unsuitable.
• Minimum Volume: 1 mL
• Reference Range: Not applicable
• Turnaround Time: 7 days
• Method: Siemens Immulite
• Quality Assurance: UK NEQAS

Anti-Mullerian hormone (AMH) is a protein produced by granulosa cells of the ovaries in females and by Sertoli cells of the testes in males.

In women serum AMH concentration increases with age up until the mid-twenties, after which it begins to decline. AMH correlates well with the number of follicles in the ovary (as measured by ultrasound) in women over the age of 25 and its measurement is used to individualise fertility treatment.

In men serum AMH concentration tends to be high in childhood, then declines through puberty to low levels in adulthood. It is used in the investigation of cryptorchidism and anorchidism.

AMH is elevated in the majority of patients with granulosa cell tumours and may be used to monitor disease progression or recurrence. AMH is also useful in the investigation of disorders of sex development as a marker of testicular activity.

Sample Requirements and Reference Ranges

• Sample Type: Serum
• Container: SST
• Precautions: Separate serum and send via first class post. If there will be >48 h before sending store the specimen at -20°C. Sample can be sent at ambient temperature.
• Minimum Volume: 2 mL
• Reference Range:
  • Females: <50 pmol/L in young adults (falls steadily towards menopause where it becomes undetectable)
  • Males (Levels fall at puberty. These ranges were derived from a study where stage of puberty was not determined):
    • 0-1yr                390-1300 pmol/L
    • 1-4yr                300-1700 pmol/L
    • 5-8yr                260-1200 pmol/L
    • 9-12yr              100-1000 pmol/L
    • 13-16yr            40-560 pmol/L
    • Adults             <100 pmol/L (literature value)

• Turnaround Time: 14 days
• Method: Beckman Access
• Quality Assurance: UK NEQAS

Growth hormone (GH) is a peptide hormone secreted by the anterior pituitary. Its main action is to stimulate the production and release of insulin-like growth factor 1 (IGF-1) by the liver. Excessive secretion causes acromegaly, while deficiency causes failure of growth in children and metabolic problems in adults.

The secretion of GH is very episodic, so random measurement is rarely useful diagnostically.

Failure of GH to suppress during a glucose tolerance test is diagnostic for acromegaly.

Stimulation tests, such as an insulin tolerance test (NB. potentially dangerous, should only be carried out in centres experienced in it) or stimulation with arginine, GHRH/arginine or glucagon, can be carried out to test for insufficiency. GH deficiency may occur as part of a more general deficiency of pituitary hormones, so other hormones are often measured at the same time.

Sample Requirements and Reference Ranges

• Sample Type: Serum
• Container: SST
• Precautions: None
• Minimum Volume: 2 mL
• Reference Range:
  • Random GH:
    • > 10 μg/L excludes GH deficiency
    • < 0.4 μg/L excludes acromegaly
  • Severe Growth Hormone Deficiency:
    • Adults Peak GH during ITT                           < 3 μg/L 
    • Adults Peak GH with GHRH/Arginine           < 5 μg/L 
    • Children Peak GH during provocation          < 5 μg/L 
  • GH Excess:
    • Failure to suppress during OGTT                 < 1 μg/L 
    • Mean integrated 24hr GH                             > 1.7 μg/L 

• Turnaround Time: 7 days
• Method: IDS iSYS
• Quality Assurance: UK NEQAS

Insulin-like growth factor 1 (IGF-1) is a peptide hormone, very similar to insulin. It is a major growth factor, which is synthesised by most cells and tissues. Circulating IGF-1 is produced by the liver in response to growth hormone (GH). IGF-1 concentration is increased in acromegaly, decreased in growth hormone deficiency and altered in systemic illness and malnutrition.

It is often measured along with growth hormone in the investigation of disorders of GH secretion. It is also used to monitor patients with acromegaly and those on growth hormone therapy.

Sample Requirements and Reference Ranges

• Sample Type: Serum
• Container: SST
• Precautions: None
• Minimum Volume: 2 mL
• Reference Range:

 Age (yr)      Males (μg/L)     Females (μg/L)

    <2              15 – 157            17 – 151

    2 – 4           28 – 247            25 – 198

    5 – 7           46 – 349            39 – 272

   8 – 10          67 – 442            59 – 371

  11 – 13         89 – 503            82 – 465

  14 – 16         104 – 510          97 – 502

  17 – 25         105 – 410          96 – 417

  26 – 39         81 – 249            72 – 259

  40 – 54         63 – 201            57 – 197

  55 – 65         49 – 191            43 – 170

  65+              39 – 186            35 – 168

• Turnaround Time: 7 days
• Method: IDS iSYS
• Quality Assurance: UK NEQAS

Insulin, produced by pancreatic beta cells, regulates glucose uptake and utilization and is involved in protein synthesis and triglyceride storage. It is often measured alongside C-peptide.

Clinical uses of insulin measurements:

1. Evaluation of possible insulinoma: In cases of hypoglycaemia, diagnosis of insulinoma relies on proving inappropriate secretion of insulin during a hypoglycaemic episode.
2. Hypoglycaemia of infancy due to hyperinsulinaemia.
3. Diagnosis of factitious hypoglycaemia together with measurement of C-peptide.
4. Discrimination of type 1 and type 2 diabetes mellitus: Insulin and C-peptide concentrations are generally low in patients with type 1 diabetes mellitus, and either normal or elevated in early type 2 diabetes, and decreased in later stages.

Sample Requirements and Reference Ranges

• Sample Type: Plasma
• Container: Lithium heparin
• Precautions: Collect after overnight fast or during symptomatic hypoglycaemia, together with glucose sample. Separate and freeze plasma within 4 hours of sample collection. Transport frozen. Haemolysed specimens unsuitable. For hypoglycaemic screen, only measure when hypoglycaemic (glucose <2.6 mmol/L). 
• Minimum Volume: 2 mL
• Reference Range: Not applicable
• Turnaround Time: 7 days
• Method: Abbott Alinity
• Quality Assurance: UK NEQAS

Insulin C-peptide (connecting peptide), a 31 amino acid polypeptide, represents the midportion of proinsulin. During insulin secretion it is enzymatically cleaved from proinsulin and co-secreted in equimolar proportion with mature insulin. The half life of C-peptide is significantly longer than insulin, so it is detectable in higher concentrations and the level less variable. C-peptide is often a more reliable marker than insulin. In addition, insulin is destroyed by proteases in haemolysed samples, while C-peptide is not.

Clinical uses:

1. Insulinoma: Elevated C-peptide levels from increased beta-cell activity.
2. Covert self-administration of insulin: Can be virtually ruled out as cause of hyperinsulinaemia by finding elevated C-peptide.
3. Type 1 diabetes mellitus: Low C-peptide levels due to diminished insulin secretion, or suppressed as a normal response to exogenous insulin. Patients on insulin can develop anti-insulin antibodies which can interfere with insulin assay, so C-peptide can be used instead to check residual beta-cell activity.

Sample Requirements and Reference Ranges

• Sample Type: Plasma
• Container: Lithium heparin
• Precautions: Collect after overnight fast. Separate and freeze plasma. Transport frozen.
• Minimum Volume: 1 mL
• Reference Range: Not applicable
• Turnaround Time: 7 days
• Method: Siemens Immulite
• Quality Assurance: UK NEQAS

Prolactin exists in various forms including the monomeric biologically active form (23kDa) and a higher molecular weight form, bound most commonly to IgG, known as macroprolactin (>100kDa). Macroprolactin lacks biological activity but can interfere in standard prolactin immunoassays and is a “common” cause of hyperprolactinaemia (overall prevalence 1.5%). Its presence is determined by recovery of prolactin following precipitation with polyethylene glycol (PEG screening test).

Macroprolactin should be requested in cases of persistently raised prolactin >700 mU/L (on two or more occasions) in euthyroid patients and after excluding drug associated hyperprolactinaemia. PEG screening can identify macroprolactin and determine the concentration of monomeric (bioactive) prolactin, as both may coincide.

Sample Requirements and Reference Ranges

• Sample Type: Serum
• Container: SST
• Precautions: None
• Minimum Volume: 2 mL
• Reference Range:
  • Macroprolactin is reported as positive or negative based on  percentage recovery of monomeric (bioactive) prolactin after PEG  precipitation to remove macroprolactin:
    • Post-PEG recovery <40% – macroprolactin detected
    • Post-PEG recovery >60% – macroprolactin negative
    • Post-PEG recovery 40 – 60% – equivocal recovery

• Turnaround Time: 7 days
• Method: Polyethylene glycol (PEG) precipitation to precipitate macroprolactin followed by Abbott Alinity immunoanalyser to quantify monomeric prolactin.
• Quality Assurance: UK NEQAS

Parathyroid hormone (PTH), a polypeptide containing 84 amino acids, is secreted by the chief cells of the parathyroid glands. It has a molecular weight of 9.4 kDa. PTH should be measured in the investigation of unexplained hypercalcaemia or hypocalcaemia. PTH should always be interpreted in light of the serum adjusted calcium concentration and the patient’s renal function.

Sample Requirements and Reference Ranges

• Sample Type: Plasma
• Container: EDTA
• Precautions: Avoid haemolysis
• Minimum Volume: 2 mL
• Reference Range: 1.6 – 7.5 pmol/L
• Turnaround Time: 1 day
• Method: Abbott Alinity
• Quality Assurance: UK NEQAS

Renin, a proteolytic enzyme, is synthesized by the juxtaglomerular cells of the kidney and released in response to decreased blood volume, decreased blood pressure and sodium depletion. Renin stimulates aldosterone release through angiotensin intermediates, resulting in the renal retention of sodium and the excretion of potassium.

Renin is measured with paired aldosterone to calculate an aldosterone/renin ratio in the investigation of hypertension.  

Renin measurement may be useful in monitoring response to therapy in patients with Addison’s disease or congenital adrenal hyperplasia (CAH).

Beta blockers, diuretics, ACE inhibitors, angiotensin II receptor blockers, calcium channel blockers, a restricted salt diet and posture can all affect interpretation of renin results.

Sample Requirements and Reference Ranges

• Sample Type: Plasma
• Container: EDTA (Lithium heparin unsuitable)
• Precautions: Do not collect on ice. Separate and freeze plasma within 4 hours of sample collection. Transport frozen. Grossly haemolysed or lipaemic samples unsuitable. Posture and relevant drug therapies (see above) may affect interpretation of results.
• Minimum Volume: 500 μL
• Reference Range:
  • Adults (upright): <52 mIU/L
  • Infants <1 year: <450 mIU/L
  • Children 1 – 5 years: <380 mIU/L
  • Children 6 – 15 years: <125 mIU/L

• Turnaround Time: 14 days
• Method: IDS iSYS
• Quality Assurance: UKNEQAS

Sex hormone binding globulin (SHBG) is a large 80-100 kDa glycoprotein that functions to transport sex hormones around the body. It has a high affinity for 17β-hydroxy steroids such as testosterone and oestradiol. Concentrations of SHBG are influenced by many factors. SHBG will be increased by elevated concentrations of circulating oestrogens (including the oral contraceptive pill), hyperthyroidism, liver disease and excess alcohol. SHBG will be decreased by increasing body mass index, polycystic ovarian syndrome and hypothyroidism.

Sample Requirements and Reference Ranges

• Sample Type: Serum
• Container: SST
• Precautions: None
• Minimum Volume: 2 mL
• Reference Range:

Age                       Male (nmol/L)      Female (nmol/L)

 3 – 10 years              45 – 220              50 – 170

 10 – 12 years            22 – 188              38 – 129

 Adult                         13 – 70                20 – 155

• Turnaround Time: 1 day
• Method: Abbott Alinity
• Quality Assurance: UK NEQAS

Anti-thyroperoxidase (TPO) antibodies are present in 90-100% of patients with Hashimoto’s thyroiditis, the commonest cause of autoimmune hypothyroidism. Anti-TPO is measured in patients with subclinical hypothyroidism (TSH 5-12 mIU/L and FT4 within reference limits: 8-21 pmol/L) to identify those at increased risk of developing thyroid failure. The risk of developing hypothyroidism if anti-TPO is positive is approximately 5% per year.

Sample Requirements and Reference Ranges

• Sample Type: Serum
• Container: SST
• Precautions: None
• Minimum Volume: 2 mL
• Reference Range: <6 IU/L
• Turnaround Time: 1 day
• Method: Abbott Alinity
• Quality Assurance: UK NEQAS

TSH receptor antibody (TRAB) is measured if the cause of thyrotoxicosis is not clear. It is specific for Graves’ autoimmune disease of thyroid but is not present in all cases. It can be used to distinguish Graves’ disease from toxic multinodular goitre and postpartum or subacute thyroiditis. It is also measured in 3rd trimester of pregnancy, if there is a maternal history of Graves’ disease/thyrotoxicosis, to predict risk of neonatal thyroid problems. It may be helpful in cases of possible “euthyroid” Graves’ ophthalmopathy.

Sample Requirements and Reference Ranges

• Sample Type: Serum (plasma unsuitable)
• Container: SST
• Precautions: Grossly lipaemic samples unsuitable
• Minimum Volume: 2 mL (Neonatal samples: minimum serum volume 200 µL)
• Reference Range:
  • <3.1 U/L – negative
  • ≥3.1 U/L – positive

• Turnaround Time: 14 days
• Method: Abbott Alinity
• Quality Assurance: UK NEQAS