Hypothyroidism (Levothyroxine) Calculator
(Adults)
|
|
BETA version
|
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
------------------------------------------
Drugs that block absorption
------------------------------------------
Cholestyramine
Sucralfate
Aluminum hydroxide
Ferrous sulfate
------------------------------------------
Drugs that increase nondeiodinative T4 clearance
------------------------------------------
Rifampin
Carbamazepine
Possibly phenytoin
------------------------------------------
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:
https://thyroidguidelines.net/hypothyroidism/guidelines.
Accessed April 2011.
[4 ] Levy EG, Ridgway EC,
Wartofsky L. Algorithms for diagnosis & management of thyroid
disorders.
https://www.thyroidtoday.com/ExpertOpinions1.asp ,
https://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. |
