Thyroid function tests

elevated FT₃ and T₃

normal FT₄

< 5% of hyperthyroid individuals --> T_{3 toxicosis}

 Critically ill, elderly, hospitalized Patient

TSH = nml or decreased (5-9%of cases)

T₃ decreased

Euthyroid sick syndrome

elevated rT₃

T₄ --> T₃ + rT₃

Hyperthyroid Patient with normal/decreased RAIU

DDx

TRH stimulation test:

Exaggerated secretion of TSH

1^o hypothyroidism due to intrinsic thyroid hypofunction

TRH -->⁺ TSH , GH, prolactin

Testing Strategy for:

Pregnancy, neonates, inpatients wit comorbidities

Graves disease >95% --> TSI/LATS Ab's -->⁺Rc-TSH

~60% --> antimicrosomal Ab's

or

~30%--> anti-thyroglobulin Ab's

low TSH

normal FT₄

additional test?

low TSH

normal FT₃ & FT₄

mild/subclinical hyperthyroidism

vs.

medication effect

Hashimoto thyroiditis

others: lymphocytic/granulomatous thyroiditis, I-131 Rx, Radiation, Drugs(Iodine,Li,IL-2,a-IFN)

Refetoff syndrome (rare)

Anti-microsomal and anti-thyroglobulin Ab's

--> a/w ?

Hashimoto thyroiditis

>90%

Pt with elevated TSH

normal FT₄

normal/low TSH

low FT₄

what is the next test?

r/o anterior pituitary pathology

complete evaluation of pituitary gland

Euthyroid Pt

normal TSH

elevated total thyroxine (T₃ + T₄)

Increase circulating TBG

pregnancy / exogenous estrogen (OCP)

--> direct thyroid toxicity and [Iodine]

iodine-rich--> hypothyroidism

iodine-poor--> hyperthyroidism 

Hyperthyroidism

[Cholesterol]_serum=?

autosomal dominant

--> peripheral hormone resistance

Normal neonate

TSH

T_4serum,FT3 , FT₄

@Delivery--> neonatal hypothyroidism =>

TSH peak @ ~ 1hr post delivery*

=> peak  T_4serum,FT3 , FT₄ --> 24-36hrs

*preterm/sick neonates => smaller peaks/delayed TSH response