FT4 level	Serum TSH reference range (0.45 to 4.50 μU per mL [0.45 to 4.50 mU per L])	Increased serum TSH (> 4.50 μU per mL)	Decreased serum TSH (<0.45 μU per mL)
Normal	Normal, euthyroid sick syndrome	Subclinical hypothyroidism	Subclinical hyperthyroidism
Increased	Early thyroiditis	Hyperthyroidism (TSH-producing pituitary adenoma)	Hyperthyroidism (factitious/iatrogenic, Graves’ disease, toxic nodule)
Decreased	Late thyroiditis	Hypothyroidism (primary thyroid failure)	Hypothyroidism (primary pituitary failure)