T4-binding globulin (TBG): factors that alter binding in serum (ATA/AACE [Garber 2012]):
Note: T4 is ~99.97% protein bound. Factors that alter protein binding will affect serum total T4 levels; however, measurement of serum free T4 (the metabolically active moiety) has largely replaced serum total T4 for thyroid status assessment.
Conditions/states that increase TBG binding: Pregnancy, hepatitis, porphyria, neonatal state
Medications that increase TBG binding: Estrogens, 5-fluorouracil, heroin, methadone, mitotane, perphenazine, selective estrogen receptor modulators (eg, tamoxifen, raloxifene)
Conditions/states that decrease TBG binding: Hepatic failure, nephrosis, severe illness
Medications that decrease TBG binding: Androgens, anabolic steroids, glucocorticoids, L-asparaginase, nicotinic acid
Thyroxine (T4) and Triiodothyronine (T3): Serum binding inhibitors (ATA/AACE [Garber 2012]):
Medications that inhibit T4 and T3 binding: Carbamazepine, furosemide, free fatty acids, heparin, NSAIDS (variable, transient), phenytoin, salicylates
Thyroid gland hormone: Interference with production and secretion (ATA/AACE [Garber 2012]):
Medications affecting iodine uptake: Amiodarone, iodinated contrast agents, iodine, ethionamide
Medications affecting hormone production: Amiodarone, ethionamide, iodinated contrast agents, iodine, sulfonylureas, sulfonamides, thionamides (carbimazole, methimazole, propylthiouracil)
Medications affecting secretion: Amiodarone, iodinated contrast agents, iodine, lithium
Medications inducing thyroiditis: Alemtuzumab, amiodarone, antiangiogenic agents (lenalidomide, thalidomide), denileukin diftitoxin, interferon alpha, interleukins, lithium, tyrosine kinase inhibitors (sunitinib, sorafenib)
Medications potentially causing the development of Graves’: Alemtuzumab, interferon alpha, antiretroviral therapy
Medications potentially ameliorating thyroiditis (if autoimmune) or Graves’: Glucocorticoids
Hypothalamic-pituitary axis and TSH: Interference with secretion (ATA/AACE [Garber 2012]):
Medications decreasing TSH secretion: Bexarotene, dopamine, dopaminergic agonists (bromocriptine, cabergoline), glucocorticoids, interleukin-6, metformin, opiates, somatostatin analogues (octreotide, lanreotide), thyroid hormone analogues
Medications increasing TSH secretion: Amphetamine, interleukin 2, metoclopramide, ritonavir, St John's wort
Medications potentially causing hypophysitis: Ipilimumab