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

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

Besides the calculator's result - the starting dose of levothyroxine - you will find useful notes on subclinical hypothyroidism on this page. We've also prepared information 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 medicine correctly everyday.

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 where the thyroid gland - a small, two-lobed gland located at the front of the neck - cannot produce a sufficient hormones to meet the body's metabolic demands.

There are two main thyroid hormones that regulate many key physiological processes in almost all of our tissues and organs. They are:

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

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

The pituitary gland controls the thyroid gland using TSH - thyroid-stimulating hormone, which tells 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, it doesn't produce enough hormones (e.g., in Hashimoto disease), so the pituitary gland answers by releasing more TSH - this is like an order to "produce more thyroid hormones!". This means that levels of TSH are 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 the basic screening indicator for thyroid malfunction.

Hypothyroidism symptoms and causes

As we said above, thyroid hormones act all over our body. Therefore, when they're in short supply, the whole body will suffer. An underactive thyroid (another name of hypothyroidism) is also a variable disease - its clinical manifestations vary depending on the patient's age, the size of the deficiency, the development of the dysfunction, any other diseases, and personal conditioning.

Hypothyroidism symptoms include:

  1. Fatigue, greater need for sleep, weakness;
  2. Cold intolerance;
  3. Weight gain;
  4. Difficulties in concentration, 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 - a slow heart rate;
  12. Hyponatremia; and
  13. Dyslipidemia - increased LDL cholesterol level, increased TG (triglycerides) level.

Judging by that enormous list of symptoms and signs, it's clear that thyroid gland failure can be a severe disease. But what causes an underactive thyroid?

First, it's worth mentioning that since the thyroid gland is so precisely controlled by 'upper floors' (i.e., the pituitary gland and hypothalamus), pathology of the thyroid's masters can cause thyroid dysfunction. Still, these situations are relatively rare, and most 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 hypothyroidism inevitable;
  • Radiation treatment - radiation in the neck, e.g., for treating lymphoma, may have a side effect such as thyroid destruction;
  • Inflammation of the thyroid; and
  • Congenital thyroid defect - for example, in babies born with genetic disorders.

What is levothyroxine?

Levothyroxine in just an analog of natural thyroxine, manufactured in laboratories as a thyroid medicine. One of its most important components 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 due to their effect on public health, efficacy, and cost-effectiveness analysis. And, most importantly, they are evidence-based and safe.

Hypothyroidism treatment. Starting dose of levothyroxine, levothyroxine side effects

It's luck that, even though an underactive thyroid is such a multidirectional disease, treating hypothyroidism is easy, cheap, and obvious - you just have to replace the lacking endogenous hormones with synthetic ones. Properly led therapy with just one daily dose of levothyroxine kind of cancels out all of 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 dose 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 your last meal). You should avoid taking iron or calcium supplements within four hours of the levothyroxine dose, as 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've settled on your levothyroxine dosage, you should get your 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, or if you get pregnant or are planning on a pregnancy.

Levothyroxine dosage calculator use

Using the levothyroxine dosage calculator is simple and intuitive. It will help you determine the right dose of the drug and give you 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 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 are dealing 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 - the dose that your patient should receive. Remember that some patients need to get there 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 no earlier than 6-8 weeks after treatment initiation or a levothyroxine dosage change.

Subclinical hypothyroidism

Subclinical hypothyroidism is the main variation of the disease, where we observe a 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 from 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.

In that case, whether or not to start levothyroxine treatment should be considered every day. Another group of patients requiring this evaluation are pregnant women or women planning a baby.

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

Aleksandra Zając, MD