Dr. Vijay Marakala, MBBS, MD.Senior LecturerBIOCHEMISTRY

IMS, MSU.

THYROID FUNCTION TESTS

THYROID FUNCTION TESTS

Introduction Indications Assay of Hormones

Radio Iodine Uptake

Detection of Thyroid

Antibodies

THYROID FUNCTION TESTS

THE THYROID GLAND IS A BUTTERFLY-

SHAPED ENDOCRINE GLAND THAT IS

NORMALLY LOCATED IN THE LOWER

FRONT OF THE NECK

THYROID FUNCTION TESTS

Laboratory determinations of thyroid function are useful in distinguishing patients with euthyroidism (normal

thyroid gland function) from those with hyperthyroidism (increased function) or hypothyroidism (decreased

function)

DISEASES OF THYROID GLAND

Hypothyroidism Hyperthyroidism

SIGNS AND SYMPTOMS OF HYP0THYROIDISM

Cold intolerance, Bradycardia

Depression

Fatigue, Goiter Hyperlipidemi

a

Weight gain

Dry skin and dry hair,

Myxedema

Menstrual irregularities

SIGNS AND SYMPTOMS OF HYPERTHYROIDISM

Heat intolerance

Fine resting tremor

Tachycardia Palpitations

Anxiety Sleep

disturbance

Irritability

Fatigue

Menstrual irregularitie

s

THYROID FUNCTION TESTS-INDICATIONS

Suspicion of thyroid disease based on clinical signs and

symptomsScreening for

thyroid disease

Evaluation of treatment for thyroid

disease.

BLOOD TESTS TO EVALUATE THYROID FUNCTION

Thyroid-stimulating hormoneTSH

Serum total thyroxineT4

Serum total triiodothyronineT3

Free thyroxineFT4

Free T3FT3

Antithyroid peroxidase antibodies, Antithyroglobulin antibodiesThyroid

antibodies

TSH : THYROID-STIMULATING HORMONE

Elevated TSH (>5 mU/L

hypothyroidism Low TSH (<0.5 mU/Lhyperthyroidism

Normal range: 0.5 – 5mU/L

One of the best thyroid function screening test

T4:SERUM TOTAL THYROXINE

Hypothyroidism• Decreased T4

Hyperthyroidism• Increased T4

Normal range: 65 – 150nmol/L

T3: SERUM TOTAL TRIIODOTHYRONINE

Hypothyroidism• Decreased T3

Hyperthyroidism• Increased T3

Normal range: 1.8 – 3nmol/L

FT4: FREE THYROXINE

Is the metabolically active thyroid hormone–not bound to protein

Should be ordered when TSH is abnormal to determine thyroid hyperfunction or

hypofunction

Normal range: 10 - 23 pmol/L

FT3: FREE TRIIODOTHYRONINE

In nonthyroidal illness, a low FT3 level is a nonspecific finding.

FT3 is done to rule out T3 toxicosis

Normal Adults: 4.0–7.4 pmol/L

FREE THYROXINE INDEX (FTI) The free thyroxine index (FTI) is a mathematical calculation

used to correct the estimated total T 4 for the amount of TBG present.

Make a calculation based on results of T 3 uptake and T 4 total, as follows: FTI= Total T4XT3 UPTAKE(%)/100

Normal Adults: 1.5–4.5 index

METHODS USED TO MEASURE THYROID HORMONES

• RadioimmunoassayRIA• Enzyme-linked immunosorbant

assayELISA• Chemiluminescent immunoassayCLIA• Fluorescent immunoassayFIA

THYROID ANTIBODIES

Antithyroid peroxidase antibodies

Used to diagnose suspected Hashimoto’s thyroiditis in

hypothyroidism

Antithyroglobulin antibodies

Used to diagnose autoimmune thyroiditis or Graves’ disease in

hyperthyroidism

TRH RESPONSE TESTTRH RESPONSE TEST

TRH administration stimulate production of TSH

If hypothalamo-pituitary-thyroid axis is normal, T3 and T4 secretions will be increased

TRH RESPONSE TESTIn primary

hyperthyroidism

• little or no change in TSH levels

In primary hypothyroidism

 • Prompt

increase in TSH

In secondary hypothyroidism• Does not

produce an increase in TSH

RADIOACTIVE IODINE UPTAKE131I used to detect

functional derangements of

thyroid gland.About 15 mci of 131I given intravenousl

y After a few hours, the patient is monitored at the

neck region by movable gamma-ray counter, which will pick up the radiation emitted by thyroid

gland

RADIOACTIVE IODINE UPTAKE

Normal response• About 25% uptake by

thyroid within 2 hours• About 50% uptake by

thyroid within 24 hours

Abnormal response• Increased uptake in

hyperthyroidism• Decreased uptake in

hyporthyroidism