Thyroid Function Tests

(TFT)

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Rationale

-  Assess for existence & indicate cause of hyperthyroidsm or hypothyroidism

-  Assess thyroid status in pts w Goitre

-  Monitor effects of thyroid medication

-  Screeing of newborn infants for congenital hypothyroidism

 

Tests

-  Thyroid Stimulation Hormone (TSH)

-  Free T4 (Thyroxine, FT4)

-  Serum T3 (Triodothyronine)

 

Features

-  TSH usually asked for singly unless:

-  Hospital in-pt

-  Suspected pituitary dysfunction

-  Psychiatric illness or dementia

-  Amenorrhoea or infertility

-  Mx w drugs likely to effect TFT

-  Glucocorticoids, Amiodarone, Phenytoin, Lithium, Carbemazepine, Dopamine

-  Levels are lab dependant

 

DDx

-  Sick Euthyroid Syndrome

 

Procedure

-  Venepuncture: orange tube

-  Guthrie Heel Prick Test for congenital hypothyroisidm

 

Interpretation

-  TSH 0.02-4 + FT4 11-24 = sick euthyroid syndrome

-  TSH < 0.02

-  + FT4 > 21 = hyperthyroidsm

-  + FT4 11-21 = ? T3 toxicosis

-  High TSH

-  + Low FT4: 1o hypothyroidism

-  + Normal FT4: subclinical hypothyroidism

-  + High FT4: 2o hyperthyroidsm (hyperpituitary)

-  Low TSH

-  + Low FT4: 2o hypothyroidism (hypopituitary)

-  + Normal FT4: subclinical hyperthyroididm

-  + High FT4: 1o hyperthyroidsm

 

 

 

Thyroid Stimulating Hormone (TSH)

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-  Best singe intial test to evaluate thyroid function

-  N = 0.3-4.0: thyroid disease unlikely

-  If abnormal (as below) take FT4

-  <: hyperthyroidsm

-  DDx drug effect: Glucocorticoids, Dopamine

-  >: hypothyroidism

-  DDx drug effect: Lithium

 

 

Free T4 (FT4)

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-  More important assessment of thyroid function than serum T4

-  Typically added when pituitary disease suspected in thyroid dysfunction

-  N = 9-25

-  <: hypothyroidism

-  >: hyperthyroidsm

 

 

Serum T3 (Triodothyronine)

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-  Converted form T4 to have biological effect

-  Used to determine T3 thyrotoxicosis (hyperthyroidsm)

-  Last test to become abnormal in hypothyroidism

-  N = 3.5-6.5

-  >: T3 toxicosis (hyperthyroidsm)