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Hypothyroidism (Levothyroxine) Calculator  (Adults)

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Background

This calculator provides very basic initial levothyroxine dosage and follow-up guidelines for adult patients with hypothyroidism.   

The starting dose, frequency of dose titration, and the optimal full replacement dose should be based on several key parameters including1:  
(1) Patient age -  e.g. older patients: start lower and titrate more slowly.
(2) Weight (1.6-1.7 mcg/kg lean body weight).  Note: based on available research,
       IBW is a suitable substitute for LBW.
(3) Cardiovascular status (positive history of coronary artery disease warrants initiation
      at a very low dose and very slow titration),  
(4) General health.
(5) Concomitant medications (see examples below).
(6) Severity and duration of hypothyroidism.

Consultation with an endocrinologist should be considered in the
following cases
1:   
(1) Patients less than 18 years old.  (2) Patients unresponsive to therapy.   (3) Pregnant patients.   (4) Cardiac patients.   (5) Presence of goiter, nodule, or other structural changes in the thyroid gland.  (6)  Presence of other endocrine disease.  (7) Patient's receiving amiodarone or other complicating concomitant therapy (8) Stupor, coma.


Situations Associated with Thyroid-stimulating Hormone Suppression2.
------------------------------
Physiologic states
------------------------------
-Autonomous thyroid function (nodular thyroid disease, subclinical Graves disease)
-Nonthyroidal illness.
-Chorionic gonadotropin excess (first trimester of pregnancy, hyperemesis gravidarum,
    molar pregnancy, choriocarcinoma)
-Recovery after therapy for hyperthyroidism or after painless or postpartum thyroiditis
-Central hypothyroidism

------------------------------
Pharmacologic situations
------------------------------
-Supraphysiologic levothyroxine therapy
-Glucocorticoid therapy
-Acute administration of dopamine or dopaminergic agents e.g. bromocriptine.
-Acute administration of somatostatin and somatostatin analogs e.g. octreotide.


Conditions that may Increase levothyroxine requirements2:
Malabsorption
    Gastrointestinal disorders:
     (1) Mucosal diseases of the small bowel (for example, sprue)
     (2) After jejunoileal bypass and small-bowel resection
     (3)Diabetic diarrhea
Cirrhosis
Pregnancy
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Drugs that block absorption
------------------------------------------
Cholestyramine
Sucralfate
Aluminum hydroxide
Ferrous sulfate

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Drugs that increase nondeiodinative T4 clearance
------------------------------------------
Rifampin
Carbamazepine
Possibly phenytoin

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Drugs that block T4 to T3 conversion
------------------------------------------
Amiodarone
Selenium deficiency




Conditions that may decrease levothyroxine requirements2:

Aging (65 years and older)

Laboratory Evaluation 3: "To establish the diagnosis of hypothyroidism, a serum TSH measurement and a free T4 estimate (or direct measurement) should be performed. When autoimmune thyroiditis is the suspected underlying cause, it is helpful to confirm antithyroid antibody titers, either antimicrosomal antibody (anti-TPO antibody) or antithyroglobulin antibody............"  See reference for additional info.

Inputs

Age:  4.    
Height:      Weight:  
If the patient's age is between 15 and 40 yo, is the FT4:   4.
Known or suspected heart disease?:

References:

Primary References:

[1 ] Demers LM, Spencer CA for the AACE Thyroid Task Force. American Association of Clinical Endocrinologists medical guidelines for clinical practice for the evaluation and treatment of hyperthyroidism and hypothyroidism. Endocr Pract. 2002;8:457-469.

[2 ] Mandel SJ, Brent GA, Larsen PR. Levothyroxine therapy in patients with thyroid disease. Ann Intern Med. 1993;119:492-502

Other:
[3 ] American Thyroid Association: http://thyroidguidelines.net/hypothyroidism/guidelines.  Accessed April 2011.

[4 ]  Levy EG,  Ridgway EC,   Wartofsky L.  Algorithms for diagnosis & management of thyroid disorders.     http://www.thyroidtoday.com/ExpertOpinions1.asp , http://www.thyroidtoday.com/ExpertOpinions/ThyroidDiseaseAlgorithms.pdf  (Accessed: April 2011)



Ideal Body Weight:
[5 ] Burton, M.E., Shaw, L.M., Schentag, J.J., and Evans, W.E. (editors). Applied Pharmacokinetics & Pharmacodynamics, Principles of Therapeutic Drug Monitoring.  Lippincott Williams & Wilkins, Baltimore, MD.   2006: 188.
Because lean body weight (LBW) - [Total body weight minus the weight of all body fat] is difficult to estimate, ideal body weight (IBW) frequently has been used....  

Estimated ideal body weight in (kg)
Males: IBW = 50 kg + 2.3 kg for each inch over 5 feet.
Females: IBW = 45.5 kg + 2.3 kg for each inch over 5 feet.


[6 ]  Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm 1974;8:650-655.

[7 ] Murphy JE. Clinical pharmacokinetics, 4th ed. ASHP, 2008:250, 397.

[8 ] Khan MA, Reddy IK. Pharmaceutical and clinical calculations. CRC Press, 2000: 268.
Disclaimer
The authors make no claims of the accuracy of the information contained herein; and these suggested doses are not a substitute for clinical judgment. Neither GlobalRPh Inc. nor any other party involved in the preparation of this program shall be liable for any special, consequential, or exemplary damages resulting in whole or part from any user's use of or reliance upon this material.  PLEASE READ THE DISCLAIMER CAREFULLY BEFORE ACCESSING OR USING THIS SITE. BY ACCESSING OR USING THIS SITE, YOU AGREE TO BE BOUND BY THE TERMS AND CONDITIONS SET FORTH IN THE DISCLAIMER.   Read the disclaimer
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