Mechanism of Action
- Synthetic form of thyroxine (T4) converts to its active metabolite, L-triiodothyronine (T3)
- Manage thyroid cancer
- Synthrox, Synthoid, Tirosint, Levoxyl, Unithroid, Levo-T, Levothroid, Eltroxin, Euthyrox, Letrox, Levaxin, Lévothyrox, L-thyroxine, Thyrax, and Thyrax Duotab, Thyrox, Thyronorm, Unithroid, Eutirox, Tirosint, Thyrin, Thyrolar
Effect at Exercise and Rest
- Increase Heart Rate (ACSM 2013)
- May provoke arrhythmias (ACSM 2013)
- Blood pressure
- Increased blood pressure (ACSM 2013)
- Slight decrease or normalization of blood pressure (Dernellis & Panaretou 2002)
- No change in exercise capacity unless angina worsens during exercise (ACSM 2013)
- No change of performance or recovery (ACSM 2013)
- Increase of cardiopulmonary reserve (ACSM 2013)
- Lower oxygen uptake and heart rate at submax cardio exercise in patients with subclinical hypothyroidism (elevated THS but normal T4) (Mainenti 2009)
Drugs Which May Inhibit T4 Conversion to T3
- Beta blockers
- Oral contraceptives
- Testosterone Replacement Therapy
- Corticosteroids (Prednisone, Hydrocortisone, etc)
- Theophylline (Bronchodilator)
- Tricyclic antidepressants
- Lithium (psychiatric medication)
- Phenytoin (anticonvulsant)
- Salicylates (anti-Inflammatory)
- Sulfonamides (antimicrobial)
Drugs / Nutrients That Impair Absorption of Thyroxine Medication
- Anti-anxiety drugs
- Arthritis medications
- Aluminum-containing products
- Such as sucralfate, antacids, and didanosine
- Sodium polystyrene sulfonate
- Resin binders
- Calcium carbonate
American College of Sports Medicine (2013). Guidelines for Exercise Testing and Prescription, William & Wilkins, 9, 405.
Dong BJ (2000). How medications affect thyroid function. Medicine Cabinet. 172(2): 102–106.
Dernellis J, Panaretou M. (2002). Effects of thyroid replacement therapy on arterial blood pressure in patients with hypertension and hypothyroidism. Am Heart J;143(4):718-24.
Haugen BR (2009). Drugs that suppress TSH or cause central hypothyroidism, Best Pract Res Clin Endocrinol Metab. 23(6), 793–800.
Mainenti MR1, Vigário PS, Teixeira PF, Maia MD, Oliveira FP, Vaisman M (2009). Effect of levothyroxine replacement on exercise performance in subclinical hypothyroidism, J Endocrinol Invest, 32: 470.
Sauvage MF, Marquet P, Rousseau A, Raby C, Buxeraud J, Lachâtre G (1998). Relationship between psychotropic drugs and thyroid function: a review. Toxicol Appl Pharmacol. 1998 Apr;149(2):127-35.