Levothyroxine Dosage Calculator

Created by Aleksandra Zając, MD
Reviewed by Dominik Czernia, PhD candidate and Jack Bowater
Last updated: Feb 15, 2022

Managing hypothyroidism treatment is a widespread challenge among physicians, and this levothyroxine dosage calculator was created to offer some help.

Besides the pure calculator result - starting dose of levothyroxine - you will find useful notes on subclinical hypothyroidism on this page. We've also covered notes for the patient, answering questions like what is hypothyroidism, what is levothyroxine, what are hypothyroidism symptoms, why it should be treated, and how to take their everyday medicine correctly.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. If any health condition bothers you, consult a physician.

What is hypothyroidism?

Hypothyroidism is a state when the thyroid gland - a small, two-lobed gland located at the front of our neck - cannot produce a sufficient amount of its hormones to meet the body's metabolic demands.

Two main thyroid hormones regulate many key physiologic processes in almost all the tissues and organs of our body:

  • thyroxine (T4);
  • thriiodothyronine (T3).

(there is also calcitonin, but it doesn't play a big role in metabolism and we're not going to describe it here)

The pituitary gland controls the thyroid gland by the TSH - thyroid-stimulating hormone, which stimulates the thyroid to produce hormones. The pituitary gland is controlled by the hypothalamus, which produces TRH - thyrotropin-releasing hormone to stimulate the pituitary gland.

scheme of thyroid system functioning - thyroid gland, pitiuary gland and hypothalamus shown

When the thyroid gland is dysfunctional and doesn't produce enough hormones (e.g., in Hashimoto disease), the pituitary gland answers releasing more TSH - this is like an order "produce more thyroid hormones!", so it's level is high while T3 and T4 levels are low. Because of that, and because measuring TSH is easier than measuring T3 and T4, we use TSH as a basic screening indicator for thyroid malfunction.

Hypothyroidism symptoms and causes

As we said above, thyroid hormones act in all over our body. Therefore, while they're in short supply, the whole organism will react, and the signs and symptoms will come from all the body systems. An underactive thyroid (another name of hypothyroidism) is also a variable disease - its clinical manifestation varies depending on the patient's age, size of deficiency, dysfunction's development, other diseases, and personal conditioning.

The hypothyroidism symptoms include:

  1. Fatigue, enlarged need for sleep, weakness;
  2. Cold intolerance;
  3. Weight gain;
  4. Difficulties in concentration, even memory impairment;
  5. Depression;
  6. Dry skin, hair thinning;
  7. Muscle pains, joint pains;
  8. Menstrual irregularities and infertility;
  9. In children - failure to thrive, lethargy;
  10. Diastolic hypertension;
  11. Bradycardia - meaning slow heart rate;
  12. Hyponatremia;
  13. Dyslipidemia - increased LDL cholesterol level, increased TG (triglycerides) level.

The list of symptoms and signs goes on and on, and it's clear that thyroid gland failure might be a severe disease. But what causes underactive thyroid?

First, it's worth mentioning that since the thyroid gland is so precisely controlled by 'upper floors' (pituitary gland and hypothalamus), the pathology of all these can cause thyroid dysfunction. Still, those situations are relatively rare, and most of the thyroid diseases begin in the gland itself. The most common causes include:

  • autoimmune disease - the patient's body produces antithyroid antibodies that attack the tissue of the thyroid - like Hashimoto disease, which is the most common cause of underactive thyroid in the United States;
  • surgical removal of the gland - e.g., because of thyroid cancer or nodules. In this case, the thyroid is simply absent, which makes clear it will cause hypothyroidism;
  • radiation treatment - radiation in the neck, e.g., for treating lymphoma, may have a side effect such as thyroid destruction;
  • inflammation of the thyroid;
  • congenital thyroid defect - for example, in babies being born with genetic disorders.

What is levothyroxine?

A short note on that: levothyroxine in just a natural thyroxine's analog, manufactured in laboratories to get a thyroid medicine. One of the most important components of it is iodium.

Levothyroxine was first synthesized in 1927 and is a proud member of WHO (World Health Organization) Model List of Essential Medicines - a list which contains drugs that are important not only for health reasons but are also essential in terms of public health, efficacy, cost-effectiveness analysis. And most important, they are evidence-based and safe.

Hypothyroidism treatment. Starting dose of levothyroxine, levothyroxine side effects

Luckily, even though underactive thyroid is such a multidirectional disease, hypothyroidism treatment is easy, cheap, and obvious - you have to replace the lacking endogenous hormones with synthetic ones. Properly led therapy with just one daily dose of levothyroxine kind of cancels all the disease's harmful effects on your body and life.

It is crucial to treat hypothyroidism and take the right levothyroxine dosage. Untreated thyroid gland failure can lead to infertility, neuromuscular dysfunctions, cognitive impairment, gathering of fluid in your pleura or pericardial sac, or even coma.

But, it is also crucial to get the levothyroxine right. You should take your medication every day, on an empty stomach, preferably 0.5h - 1h before breakfast (alternatively, you can take the dose at bedtime, at least 3 hours after last meal). You should avoid taking iron or calcium supplements within four hours of levothyroxine dose - they decrease hormone absorption.

Hypothyroidism treatment is also safe. The only possible dangers are taking too little or too much of the medication. In the first case, a patient won't feel any better, and their TSH level will still be high. That's why at the starting dose of levothyroxine, it's important to measure TSH 6-8 weeks after treatment initiation.

While taking an excessive dose, a patient will complain about shakiness, nervousness, feeling hot, diarrhea. Their TSH level will be small or even indeterminate.

Once you're settled on their levothyroxine dosage, you should get TSH measured once a year. You need an earlier test if you're not feeling well if you start or stop taking any other medications that can influence the TSH level, if you get pregnant or plan pregnancy.

Levothyroxine dosage calculator use

Using the levothyroxine dosage calculator is simple and intuitive. It will help you determine the drug's right dose and give a hint of where to start and where to aim.

  1. Choose the sex of your patient.
  2. Choose the age range of your patient. The majority of older people need smaller dosages of the medication.
  3. Fill in the weight of your patient.
  4. Input the height of your patient. You may be surprised by this, but our calculator works either on actual weight, or on an ideal body weight in case of people above their ideal weight. It uses the Devine formula, which varies depending on sex.
  5. Input the patient's current level of FT4 (thyroxine). It is needed to determine if we deal with clinical or subclinical hypothyroidism (SCH). Usually, patients with SCH require lower doses.
  6. Check the presence of heart disease in your patient.

You'll get an immediate result of the dose that your patient should receive. Remember that some patients need to get there slowly by gradually increasing the dosage.

You'll also see a short comment below on the target TSH level. Always pay attention to the laboratory working on your result, as the normal ranges may vary.
Remember to test TSH not earlier than 6-8 weeks after treatment initiation or levothyroxine dosage change.

Subclinical hypothyroidism

Subclinical hypothyroidism is the main disease's variation, where we observe high TSH level and normal T3 and T4 levels. A patient can complain of symptoms, but it is not necessary to state the diagnosis.

Not every latent thyroid gland failure needs to be treated. The chances of progressing the subclinical hypothyroidism to an overt disease grow with the initial level of TSH (especially with TSH > 10 mU/L) and the presence of specific antibodies.

Those are the situations when we should consider starting a dose of levothyroxine every day. Another group of patients requiring treatment are pregnant women or women planning a baby.

Whether treated or not, subclinical hypothyroidism is a yellow flag that should put a patient on a 'regular thyroid laboratory tests' list.

Aleksandra Zając, MD
Sex
Female
Age
15-50 years old
Weight
lb
Height
ft
in
Current level of FT4
Low
Heart disease
No
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