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 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.