Interpretation of Thyroid function

tests in PregnancyDR. JYOTI BHASKARDR. SHARDA JAINDR. JYOTI AGARWAL

NORMAL THYROID PHYSIOLOGY

NORMAL THYROID PHYSIOLOGY

Hypothyroidism Hyperthyroidism

Diseases of Thyroid Gland

Elevated TSH (>4.5 mU/Lhypothyroidism

Low TSH (<4.5 mU/Lhyperthyroidism

Normal range: 0.5 – 4.5mU/L

One of the best thyroid function screening test

TSH : Thyroid – Stimulation Hormone

Hypothyroidism• Decreased T4

Hyperthyroidism• Increased T4

Normal range: 65 – 150nmol/L

T4 : Serum Total Thyroxine

Hypothyroidism• Decreased T3

Hyperthyroidism• Increased T3

Normal range: 1.8 – 3nmol/L

T3: Serum Total Triiodothyronine

Laboratory Diagnosis TSH assay: Primary test

to establish the diagnosis

Additional tests: Estimation of free T3

and T4 Test for thyroid

autoantibodies Thyroid

scan/ultrasonography Serum cholesterol-

in hypothyroidism

T3/T4

Subclinical hypothyroidism

Overt hypothyroidism

TSH

TSH

AACE medical guidelines for clinical practice for the evaluation and treatment of hyperthyroidism and hypothyroidism, 2006:1-13http://www.aace.com/pub/pdf/guidelines/hypo_hyper.pdf.

T3/T4

Patterns of thyroid function tests during assessment of thyroid function

Source: UpToDate.com.

Serum TSH Serum Free T4 Serum T3 AssessmentNormal hypothalamic-pituitary functionNormal Normal Normal EuthyroidHigh Low Normal or low Primary hypothyroidismHigh Normal Normal Subclinical hypothyroidismLow High or normal High HyperthyroidismLow Normal Normal Subclinical hyperthyroidismAbnormal hypothalamic-pituitary functionNormal or high High High TSH-mediated hyperthyroidism

Normal or low* Low or low-normal Low or normal Central hypothyroidism

* In central hypothyroidism, serum TSH may be low, normal or slightly high.

Antithyroid peroxidase antibodies

Used to diagnose suspected Hashimoto’s thyroiditis in

hypothyroidism

Antithyroglobulin antibodies

Used to diagnose autoimmune thyroiditis or

Graves’ disease in hyperthyroidism

Thyroid Antibodies

Thyroid Physiology in Pregnancy

Decreased TSH

Placenta hCG

Increased TH degradation

Thyroid stimulation for increased TH

Production

Estrogen

Increased serum TBG

Increased hepatic production of TBG

Increased half life of TBG

Type 3 deiodinase

Stimulates TSH receptor

Decreased free TH

Stimulation of hypothalamic pituitary axisIncreased serum TH

hCG: Human chorionic gonadotropinTSH: Thyroid stimulating hormoneTH: Thyroid hormoneTBG: Thyroxine-binding globulin

Increased oestrogen in pregnancy

Two- to threefold

increase in TBG

Decrease Free T3 and T4

Similar structure of hCG and TSH

hCG stimulates

release of T3 and T4

Transient TSH decrease in

weeks 8 to 14

Increased peripheral

metabolism of T3 and T4

Decrease Free T3 and T4

Thyroid Physiology in Pregnancy

1. Jameson JL. Harrison's Principles of Internal Medicine. 2008:2224-2247.2. Lazarus JH. Br Med Bull.2010;1-12.3. Galofre JC, Davies TF. J Womens Health (Larchmt). 2009;18(11):1847-1856..

0

50

75

100

125

150

0

0.5

1.5

2.5

3.5

4.5

40 8 12 16 20 24 28 32 36 40

TSH

(mU

/L)

hCG

(IU

/mL

hCGTSH

Weeks of gestation

0

1.5

2.0

2.5

3.0

3.5

0

5

10

15

20

25

40 8 12 16 20 24 28 32 36 40

TT4

(mcg

/dL)

TBG

(mg/

dL)

TBGTotal T4

Weeks of gestation

0.2

0.6

1.0

1.4

1.8

160

220

280

340

400

40 8 12 16 20 24 28 32 36 40

FT3

(pg/

dL)

FT4

(ng/

dL)

Free T4Free T3

Weeks of gestation

A

B CTBG: Thyroxine- binding globulinTSH: Thyroid stimulating hormoneHCG: Human chorionic gonadotropinTT4: Total thyroxineFT3: Free triiodothyronineFT4: Free thyroxine

Thyroid evaluation in normal pregnancy

Recommendation Indication

TSH and FT4 ScreeningInterpretation should be trimester specific

 TPO-Ab and Tg-Ab Presence of AITD

 Ultrasound Advisable when nodular disease is suggested by clinical examination

1. Galofre JC, Davies TF. J Womens Health (Larchmt). 2009;18(11):1847-1856.

Usual recommendations for thyroid evaluation in normal pregnancy

Thyroid hormone changes during pregnancy

First trimester Second trimester

Third trimester

TSH Normal or decreased Normal Normal

Free T4 Normal Normal NormalFree T3 Normal Normal NormalTotal T4 High High HighTotal T3 High High High

What is a normal TSH level in pregnancy?

Reference range used for

nonpregnant population

First trimester Second trimester

Third trimester

FT4 (pmol/L)

9–26 (0.7–2.02 ng/dL)

10–16 (0.78–1.25 ng/dL)

9–15.5 (0.70–1.3 ng/dL)

8–14.5 (0.62–1.13 ng/dL)

FT3 (pmol/L)

2.60–5.7(0.2–0.44 ng/dL)

3–7 (0.23–0.55 ng/dL)

3–5.5(0.23–0.43 ng/dL)

2.5–5.5(0.2–0.43 ng/dL)

TSH (mu/L) 0.3–4.2 0.1–2.5 0.2–3.0 0.3–3.0

What are the Screening tests to be done?

TSH

f T4 and anti TPO

IF HIGH ( Pregnancy specific levels)

Anti TPOOver 58% of cases of SCH have anti

TPO

It is Positive only when levels are TWO times the Normal

Once positive – should not be repeated again.

Anti TPO Significance :

1. It defines the cause as Autoimmune 2. It cautions for increased chances of

PPT 3. Normal TSH with positive antibodies

are prone to become hypothyroid – Need Monitoring

Anti TPO to be done

Personal history of Thyroid or AI disorders

Family History of Thyroid diseaseWomen with GoitreWomen with history of

miscarriage or Preterm deliveryWomen with Type 1 diabetes

TAKE HOME MESSAGE

We need to understand the thyroid physiology in pregnancy before interpreting the results.

All pregnant women have to be screened for Thyroid disorders at the first antenatal visit

The screening test recommended is TSH

Other two tests that are of clinical significance are

Free T4 and Anti TPO

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