Hypothyroidism (Levothyroxine) Calculator  (Adults)

BETA version
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Actual body weight:  0.0  kg
Lean body weight (IBW) 5,6,7,8
:    kg

Initial Regimen:

Basic info: Patient is 15 to 40 years old. NO known or suspected heart disease. FT4 is low. Start patient on 0.0 mcg (1.6 mcg/kg/day) based on the lean body weight (see reference section). Repeat TSH level in 6 to 8 weeks.

Follow up (every 6 to 8 weeks initially):

TSH Result- If the level is > 4.12 mIU/L INCREASE the levothyroxine dose by 12.5 to 25 mcg per day and then repeat TSH test in 6 to 8 weeks.

TSH Result- if the level is 0.45 to 4.12 mIU/L continue current dose and repeat TSH test annually or if patient becomes symptomatic.

TSH Result- if the TSH level is less than 0.45 mIU/L, DECREASE the levothyroxine sodium dose by 12.5 to 25 mcg per day increments and then repeat TSH test in 6 to 8 weeks..

Dosage Modification (hint):

If the patient requires a dosage increase of 12.5 to  25 mcg per day, it may be possible to have the patient simply take an additional tablet once per week (note: the half-life of levothyroxine is ~ 7 days).   Example:  patient is receiving levothyroxine 100mcg daily and after 6 weeks the patient's TSH value is 8.6 mIU/L.  This particular patient can take 2 tablets one day per week (e.g. every Monday).  This represents a daily increase of ~14.3 mcg.

Alternatively, if the patient requires a dosage decrease of 12.5 to 25 mcg per day, simply have the patient skip a dose one day of the week depending on the maintenance dose. 

In other cases, the patient will require a new prescription if this strategy does not target the desired daily dose.

Package insert related output:

Replacement Therapy in Overt Hypothyroidisma
Adult Patients Starting Dose TSH Goal
Under age 50b 1.7 mcg/kg/day  [0.0 mcg per day based on lean body weight above] 1 to 2
Over age 50 25-50 mcg/day Mid-normal range
With heart disease 12.5-25 mcg/day Mid-normal range
Replacement Therapy in Subclinical Hypothyroidism

 [ Subclinical hypothyroidism algorithm ]
Adult Patients Starting Dose TSH Goal
Under age 50c 1.0 mcg/kg/day  [0.0 mcg per day based on lean body weight above] 1 to 2
Over age 50 25-50 mcg/day Mid-normal range
With heart disease 12.5-25 mcg/day Mid-normal range
Suppressive Therapy for Differentiated Thyroid Cancer
1. If the patient has persistent disease, the serum TSH should be maintained below 0.1 mU/L as long as it can be safely administered.
2. If the patient is free of disease, but at high risk for recurrence or metastasis, the TSH should be kept between 0.1 and 0.5 mU/L for five to 10 years.
3. Those at low risk and free of disease may be titrated to TSH levels in the low normal range of 0.3 to 2.0 mU/L.

a Myxedema should not be treated initially with oral replacement therapy and intravenous levothyroxine is often recommended.
b Patients under age 50 in otherwise good health and without risks for heart disease.
c Requirements for subclinical hypothyroidism are often less than the full replacement dose needed in overt hypothyroidism.

Levothyroxine Sodium Dosing (Adults)  9,10
General Principles (Package insert)
The goal of replacement therapy is to achieve and maintain a clinical and biochemical euthyroid state. The goal of suppressive therapy is to inhibit growth and/or function of abnormal thyroid tissue. The dose of SYNTHROID that is adequate to achieve these goals depends on a variety of factors including the patient's age, body weight, cardiovascular status, concomitant medical conditions, including pregnancy, concomitant medications, and the specific nature of the condition being treated. (see package insert for WARNINGS and PRECAUTIONS). Hence, the following recommendations serve only as dosing guidelines. Dosing must be individualized and adjustments made based on periodic assessment of the patient's clinical response and laboratory parameters. SYNTHROID is administered as a single daily dose, preferably one-half to one-hour before breakfast. SYNTHROID should be taken at least 4 hours apart from drugs that are known to interfere with its absorption. Due to the long half-life of levothyroxine, the peak therapeutic effect at a given dose of levothyroxine sodium may not be attained for 4-6 weeks. Caution should be exercised when administering SYNTHROID to patients with underlying cardiovascular disease, to the elderly, and to those with concomitant adrenal insufficiency.
 Review Entries [Go back]


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

[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.

Alternative dosing:
[9 ] USPharmacist:  http://www.uspharmacist.com/continuing_education/ceviewtest/lessonid/105477/
(Accessed:   April 2011)

[10 ]  Synthroid (levothyroxine sodium tablets, USP) prescribing information. North Chicago, IL: Abbott Laboratories; March 2011.  http://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?id=40193#nlm34068-7    (Accessed:   April 2011)