The Mis-Interpretation of Endocrine

Laboratory ResultsA Primer on the Thought Process of the Endocrinologist

Alexander Karmazin, MD, FAAP, FACE

Pediatric Endocrinologist

Sanford Children’s Specialty Clinic

Assistant Professor of Pediatrics

Sanford USD School of Medicine

Learning Objectives

• Identify laboratory values that might not be normal, despite being within the reference range on the laboratory report

• Identify laboratory values that might be normal, despite not being within the reference range on the laboratory report

• Recognize situations in which exam findings provide more information than laboratory test results

• Understand that age, gender, and Tanner stage may significantly influence a laboratory test’s reference range

Areas To Be Covered

• Growth

• Thyroid function

• Puberty

• Calcium/Vitamin D metabolism

Growth Evaluation

• Bone age X-rays

• Serum growth hormone

• IGF-1

• IGF BP-3

Bone Age X-Rays

• Inexpensive, informative, predictive

• Subjective and interpreter-dependent

• Should never be interpreted as either

“normal” or “abnormal”

• Cannot differentiate between certain

diagnoses – growth hormone deficiency

vs. constitutional delay of growth

Normal (bad) vs. Abnormal (good)

Growth hormone

• Random values not reliable for

diagnosis of either GH deficiency or

excess (diurnal variability)

• Usually requires prolonged sampling

under either a provocative test or

inhibitory protocol using oral glucose

IGF-1 and IGF BP-3

• Values highly dependent on age,

gender, and Tanner stage

• Regulated by growth hormone

production and nutritional intake

• Normal reference ranges, particularly in

adolescents are wide

A Clinical Example (A Recent Patient Seen in Clinic)

• 6-4/12 y.o. male referred for short stature

• Limited growth data available, but apparently no change in height in past 8 months

• BMP, CBC, TSH obtained locally all normal

• PMH: Asthma; Meds: Singulair, Flovent, Albuterol MDI PRN

• Physical exam unremarkable

• Bone age = 4 years (1 S.D. for age = 9.3 months)

• TSH = 3.38 mcIU/mL (0.4-5), free T4 = 0.9 ng/dL (0.7-1.5), sed rate = 11 mm/hr (0-22), IGF-1 = 17 ng/mL (60-228), IGF BP-3 = 1.3 mg/L (1.5-3.4), celiac screen – strongly positive

A Clinical Example (A Recent Patient Seen in Clinic)

• Why did I repeat thyroid functions when

the TSH value was normal locally?

• Does this child’s combination of bone

age delay and low IGF-1/IGF BP-3

values indicate a diagnosis of growth

hormone deficiency despite his small

bowel biopsy showing villous atrophy &

increased epithelial lymphocytes?

Thyroid Function Testing

• TSH

• Total and free T4

• Total and free T3

• T3 resin uptake (T3RU)

• Reverse T3

• Thyroglobulin

Thyroid Stimulating Hormone

• Very difficult (but not impossible) for a laboratory to perform incorrectly

• Values generally stable over the lifespan

• An excellent screen for primary hypothyroidism

• Alone, an inadequate screen for secondary or tertiary hypothyroidism

• Not useful for ongoing monitoring of patients with panhypopituitarism on thyroid replacement

• If mildly elevated (5-10 mcIU/mL), should not be blamed for rapid weight gain or other significant symptoms of hypothyroidism

Total T4

• A measure of all tetraiodothyronine in the bloodstream

• All but 99.97% is bound to proteins and thus inactive

• Dependent on thyroxine binding globulin concentration

• Values relatively stable over the lifespan, however children under 2 tend to have slightly higher reference ranges

Free T4

• Usually performed by an indirect method that

can rarely be affected by non-specific

absorption of free T4 to matrix reagents

• Can be performed by a direct method called

equilibrium (direct) dialysis

• Values from one laboratory cannot be

compared to those from another

– Reference ranges can vary from 0.59-1.17 ng/dL

to 1.1-1.8 ng/dL

Free T4, continued

• Eliminates issues of TBG deficiency and TBG excess

• Generally should be obtained with TSH in patients with primary hypothyroidism on thyroid replacement

• Can be used alone to monitor therapy for those with central hypothyroidism/ hypopituitarism

• Should make sense when interpreted together with the TSH value

Total and Free T3

• Mostly made by deiodination from T4, though some is made directly in the thyroid gland

• Values generally decrease slightly over the lifespan

• Free T3 values are not well established in the pediatric population

• Most useful in evaluation of possible pure T3 toxicosis – suppressed TSH and normal or close to normal free T4

T3 Resin Uptake

• Historically was used as an indirect measure of free T4 concentration by assessing T4 protein binding

• Says absolutely nothing about T3 concentration

• Since the development of free T4 assays, is generally relegated to the medical history books

Reverse T3

• A metabolically inert product of deiodination of T4

• Usually elevated in cases of non-thyroidal illness (euthyroid sick syndrome) as deiodination to T3 is inhibited

• Euthyroid sick syndrome can be associated with a variety of conditions– Fasting, malnutrition, trauma, surgery, severe

systemic illness, poorly controlled diabetes

• Rarely if ever ordered by pediatric endocrinologists

Thyroglobulin

• Precursor molecule for thyroid hormone

mostly stored as colloid

• Note that this is note the same as

thyroglobulin antibodies

• Mostly used for post-operative monitoring in

thyroid cancer

• Can be used to distinguish between

endogenous and exogenous hyperthyroidism

A Clinical Example

• 12 y.o. female referred for “abnormal thyroid function tests” and recent weight gain

• Locally: TSH = 6.2 mcIU/mL (0.35-4.94), free T4 = 1 ng/dL (0.7-1.5)

• Height points consistently at the 75th

percentile

• PMH: Negative; Meds: None

• FH: No family history of thyroid disease

• Exam: No thyromegaly, otherwise normal

A Clinical Example

• Should this patient be started on thyroid

replacement?

• If not, what is the next step?

• If thyroid functions should be

repeated, when?

• Would thyroid antibodies be helpful?

• Any role for thyroid ultrasound?

The Pituitary-Gonadal Axis

• FSH

• LH

• Testosterone

• Free testosterone

• Estradiol

• Other serum and urine estrogens

Gonadotropins (FSH & LH)

• Both boys and girls go through a “mini-puberty” of infancy

– LH values begin to increase two weeks after birth and decline to pre-pubertal values by 1 year of age

– FSH values also begin to increase two weeks after birth and decline to pre-pubertal levels by 1 year of age in boys and 2 years of age in girls

Gonadotropins (FSH & LH)

• Low values in the early to mid-teen

years cannot distinguish between

hypogonadotropic hypogonadism and

simple pubertal delay

• High values indicative of gonadal failure

cannot be expected until the end of the

first decade of life

Gonadotropins (FSH & LH)

• Reference ranges are available by age,

Tanner stage, and gender

• In pediatrics, testing should occur in a

laboratory with expertise in analysis of

pediatric samples and established

reference ranges for children

Testosterone

• Remember the “mini-puberty”

phenomenon

– In boys, levels are elevated (75-400 ng/dL)

at birth, decrease rapidly during the first

week (20-50 ng/dL), increase to a peak

during days 20-60 (60-400 ng/dL), then

decline to a pre-pubertal level (<10 ng/dL)

by 7 months of age

Testosterone

• Standard laboratories often cannot detect

testosterone concentrations <40 ng/dL

• Pediatric samples thus should be

processed at endocrine-specific

laboratories that can detect concentrations

as low as 3 ng/dL

• The generally accepted cut-off of pre- vs.

in puberty testosterone values is 10 ng/dL

Free Testosterone

• In most cases, offers little if any

advantage over total testosterone

• May offer greater sensitivity in the

evaluation of hyperandrogenemic girls

because androgen excess lowers levels

of circulating SHBG (sex hormone

binding globulin)

Estradiol

• The “mini-puberty” phenomenon is back one more time

– Levels are markedly elevated at birth, fall rapidly during the first week to pre-pubertal levels, increase between 30 and 60 days (1.0-3.2 ng/dL for males, 0.5-5.0 ng/dL for females), and decline to pre-pubertal levels again by six months in boys and by 1 year in girls

Estradiol

• Different laboratories (even endocrine

specific ones) may use different units of

measure for estradiol assays

– Quest Diagnositics uses pg/mL

– Esoterix uses ng/dL

– There is a 10-fold difference between them

• Low values may not be helpful in females

because of the cyclic nature of hormone

production

Other serum and urine estrogens

• In the general pediatric and pediatric

endocrinology practices, provide no

additional information or benefit in the

detailed evaluation of the pituitary-

gonadal axis

A Clinical Example

• 15-1/2 year old male presents for evaluation of delayed puberty

• No significant past medical history

• Height at the 10th percentile

• Exam shows normal pre-pubertal male genitalia with testicular volume of 3 mL

• Labs: TSH = 1.5 mcIU/mL (0.4-5); FSH = 0.65 IU/L (0.81-8.18 for 15-18 y.o. males), LH = 0.14 IU/L (0.69-7.15 for 15-18 y.o. males), testosterone = 12 mg/dL (220-880 for 15-17 y.o. males)

• What is the most likely diagnosis?

Calcium Metabolism

• Parathyroid Hormone (PTH)

• Calcium

• Ionized calcium

• Phosphorus

• Vitamin D

– 25-Hydroxy

– 1,25-Dihydroxy

Parathyroid Hormone

• Should be measured as the intact 84 amino acid molecule

• Reference range is generally unchanged over one’s lifetime

• Cannot be interpreted in isolation from calcium or ionized calcium values (many labs automatically perform a serum calcium when PTH is ordered)

Parathyroid Hormone

• Normal PTH levels in the face of

hypocalcemia are “inappropriately

normal” – i.e. LOW – and may be

indicative of either hypoparathyroidism

or a gain-of-function mutation in the

calcium sensing receptor (autosomal

dominant hypoparathyroidism)

Parathyroid Hormone

• Normal PTH levels in the face of

hypercalcemia are “inappropriately

normal” – i.e. HIGH – and may be

indicative of either hyperparathyroidism

or a loss-of-function mutation in the

calcium sensing receptor (familial

hypocalciuric hypercalcemia)

Calcium and Ionized Calcium

• Reference range is slightly higher during the first decade of life

– Total calcium• Full term-<10 days 7.6-10.4 mg/dL

• 10 days-2 years 9.0-11.0 mg/dL

• 2-12 years 8.8-10.8 mg/dL

• Adult 8.6-10 mg/dL– Source: Harriet Lane Handbook

– Ionized values show a similar pattern

Ionized Calcium

• Represents the concentration of free

and biologically active calcium in blood

• Calcium circulates in approximately

equal free and bound fractions

• Permits a more accurate assessment of

hypo- and hypercalcemic states in the

presence of abnormal albumin levels

Phosphorus

• Levels are significantly higher in the first

decade compared to later in life

– Newborn 4.5-9.0 mg/dL

– 10 days-24 months 4.5-6.7 mg/dL

– 24 months-12 years 4.5-5.5 mg/dL

– >12 years 2.7-4.5 mg/dL

• Source: Harriet Lane Handbook

Phosphorus

• Values typically low in all causes of

rickets

• Values typically inverse to those of

calcium in parathyroid disorders

(hypopara, hyperpara, pseudohypopara)

– PTH increases calcium resorption and

phosphorus excretion

25-Hydroxy Vitamin D

• The test best reflective of total body vitamin D

stores

• Sometimes reported separately as vitamin D2

(plant sources) and vitamin D3 (animal

sources and skin production)

• Optimal value is ≥32 ng/mL, no matter what

the laboratory reports as the reference range

1,25-Dihydroxy Vitamin D

• The active form of vitamin D

• Stored 25-Hydroxy Vitamin D is 1-

hydroxylated by the kidney in a tightly

regulated fashion

• Tends to be elevated in hyperparathyroid

and early rachitic states; decreased in

renal failure and late rachitic states

A Clinical Example

• An infant in the NICU has been having seizures

• She is also known to have a ventricular septal defect

• Labs– Calcium = 6.1 mg/dL (7.6-10.4)

– Ionized calcium = 0.88 mmol/L (1.1-1.42)

– Phosphorus = 8.6 mg/dL (4.5-9 for newborn, 4.5-6.7 after 10 days of age)

– PTH = 38 pg/mL (9-59)

– 25-OH vitamin D = 41 ng/mL (32-?)

• What is the most likely diagnosis?

• What other test should be obtained?

Summary

• Endocrine laboratory tests must be interpreted in light of the patients clinical presentation and physical exam findings

• Some endocrine laboratory tests cannot be interpreted in isolation from others

• Reference range based on age, gender, and Tanner stage is often vital in order to properly interpret endocrine results