Optimal TSH Levels When Treating Hypothyroidism

The thyroid stimulating hormone (TSH) is the blood test used for both diagnosing and managing hypothyroidism. However, interpreting a person's TSH result can be a bit tricky, as there is some debate about what exactly a "normal" and "optimal" TSH level is.

What Is a "Normal" TSH Level?

For most laboratories, a "normal" TSH level is around 0.4 or 0.5 to 4.5 or 5.0 milli-international units per liter (mIU/L). However, there is debate amongst experts regarding this range. 

  • For instance, some experts argue that the upper limit of a normal TSH should be lower (around 2.5 mIU/L), due to the fact that the vast majority of young, euthyroid adults have a TSH value between 0.4 and 2.5 mIU/L.

The problem with changing this range is that it would mean starting many more people on thyroid hormone replacement medication.

In addition, many doctors follow age-adjusted TSH levels, meaning they believe that the upper limit of normal for an older person should be higher than 4.5 to 5.0 mIU/L. This is because as people get older, their TSH's naturally rise.

What Is an "Optimal" TSH Level?

In a person diagnosed with primary hypothyroidism, deciding their optimal TSH level requires some careful thought.

While one would think that achieving a TSH within the "normal" reference range is ideal, there are actually other goals of therapy (besides hitting a target TSH level).

These goals of treatment include:

  • Improvement of symptoms
  • Reduction in any thyroid enlargement, called a goiter (if present)
  • Avoiding overtreatment (called iatrogenic thyrotoxicosis)

For example, if a person has a TSH that falls within the upper limits of normal, and they have hypothyroid symptoms (for example, constipation or cold intolerance), a doctor may increase the dose of the thyroid hormone replacement medication with the goal of targeting a lower TSH (one that is closer to 1.0 mIU/L).

This is a sensible approach as it's a way of treating the patient, not just a number.

Even so, you may wonder why doctors do not always target a TSH at the lower end of "normal."


The truth is that for certain people, a lower TSH level (one that is between 0.4 to 2.5 mIU/L) is optimal in young and middle-aged people. This is because research has shown that in the general population, nine out of ten people have a TSH level of 1.4 mIU/L. 

But in older people (age 65 or 70 and older), a target TSH of 3.0 to 6.0 mIU/L is more appropriate, considering the natural age-related rise in TSH. 

Health Risks

Besides considering age, a doctor may also consider your underlying medical history (like if you have heart problems), as there are health risks associated with over-treating a person's thyroid.

In fact, over replacing your thyroid hormone can trigger a heart condition called atrial fibrillation (this more commonly occurs in older people) and/or bone loss (this is especially concerning for postmenopausal women). 

A Word From Verywell

Once you have been diagnosed with hypothyroidism, determining your optimal or target TSH level can take time and patience as you and your doctor sort out various factors like your symptoms, your overall health, and your age.

In the end, be at ease that you can find the right dose of medication to manage your hypothyroidism. Remain focused on your well-being and your healthy habits as you navigate through your thyroid journey.

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Article Sources
  • Braverman, L, Cooper D. Werner & Ingbar's The Thyroid, 10th Edition. WLL/Wolters Kluwer; 2012.
  • Jonklaas, J et al. "Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement (2014)." Thyroid 24(12): 1670-1751, 2014.
  • Ross DS. (2018). Treatment of primary hypothyroidism in adults. Cooper DS, ed. UpToDate. Waltham, MA: UpToDate Inc.