_↓T_{4 & ↑ TSH --> primary hypothyroidism}

↓ T4 & ↓ TSH _--> central hypothyroidism

↑ T4 & ↑ T3 _-->yperthyroidism (90% of cases)

nml or ↓ T4 & ↑ T3 _--> hyperthyroidism (5% of cases)

↓ T4 & nml or ↓ T3 & nml TSH _--> consider euthyroid sick syndrome (the T4 should go back into the normal range after resolution of acute nonthyroidal illness); also have increased levels of reverse T3 . This is due to decreased conversion of T4 to T3 and decreased conversion of rT3 to T2. The finding of an elevated reverse T3 in the critically ill patient helps exclude a diagnosis of hypothyroidism. (TSH will remain essentially normal)

↑ T4 & nml or ↓ T3 _--> T4 toxicosis (should usually have a rise in T3 over time into the toxic range).

nml or ↑ TSH & ↑ serum freeT4 _--> inappropriate secretion of TSH (can be due to changes in protein-binding antibodies to T4, or TSH, or drugs that inhibit the conversion of T4 to T3, or due to a TSH-producing pituitary tumor and selective pituitary resistance).

↑ T4 may be due to drugs that increase total Thyroid-Binding Globulin (e.g., estrogen, heroin); will have a normal FT4I

↓ T4 while patient is clinically euthyroid can be seen with hypoproteinemia due to malnutrition (have deficiency of thyroid-binding proteins and thus low total T4 ), protein-losing enteropathies, renal dialysis, nephrotic syndrome.

CHLA Board Review 2005