Increased In:

• Conditions with increased TBG (e.g., pregnancy and estrogen therapy)
• Early thyroid failure
• Hyperthyroidism (triiodothyronine is produced independently of stimulation by TSH)
• Iodine-deficiency goiter
• T₃ toxicosis
• Thyrotoxicosis factitia (laboratory tests do not distinguish between endogenous and exogenous sources)
• Treated hyperthyroidism

Decreased In:

• Acute and subacute nonthyroidal disease (pathophysiology is unclear)
• Conditions with decreased TBG (TBG is the major transport protein)
• Hypothyroidism (thyroid hormones are not produced in sufficient quantities regardless of TSH levels)
• Malnutrition (related to insufficient protein sources to form albumin and TBG)