Levothyroxine Dosing: The Evidence Behind the Tool

A 68-year-old retired headmistress from Kundapura comes for her yearly thyroid check. She weighs 84 kg and has taken levothyroxine 150 mcg every morning for twelve years. She feels well. Her TSH is 0.06 mIU/L. Her daughter has read that the dose should go up with weight, and asks whether 150 mcg is now too little. This is an illustrative case, not a real patient.

The Levothyroxine Dosing tool works out the dose, the target and the recheck interval. It keeps every dose, every target and every caution. What it no longer carries is some of the why: why weight misleads, why the target moves with age, and the studies behind two less common situations. Those have moved here, so that the tool stays quick at the bedside. By the end you will know why the daughter’s question has the opposite answer to the one she expects.

1. Why weight alone misleads

The textbook dose is 1.6 to 1.8 mcg/kg of actual body weight. That works well in a lean patient. It works less well as weight rises, and here is why.

“Surely a heavier patient needs more hormone?” Only up to a point. Deiodination happens in muscle, liver and skin, not in adipose tissue, so extra fat adds dose without adding capacity to use it. Thyroxine is turned into its active form in lean tissue. Fat adds kilograms to the calculation, but it adds very little that uses the hormone.

This is why the tool computes the dose two ways, from actual body weight and from lean body mass, and shows both. In an overweight patient the two answers can be more than one tablet step apart. Our headmistress weighs 84 kg. A dose worked out on her actual weight alone would be larger than her body can use.

2. The target rises with age

Her TSH is 0.06 mIU/L. Before we decide anything, we should ask what a normal TSH is at 68.

The TSH distribution shifts right with age in people who have no thyroid disease. The 97.5th centile in healthy elderly people is 7.5 mIU/L. In plain words: a TSH that would look high in a young adult is quite usual in a healthy older person. That is why the 2014 guideline calls it reasonable to aim higher, at 4 to 6 mIU/L, above 70 to 80 years, and why the national workflow prints its own bands that rise with age. The tool reads those bands for you.

The common mistake is to aim for the same TSH at every age. It is easy to see why. One reference range is printed on the report, whatever the patient’s age. But a range drawn from younger adults does not describe an 80-year-old.

3. Overtreatment is the harm we cause

“She feels well. Does a low TSH matter?” It does. Above 65 years, a TSH below 0.1 mIU/L carried a threefold increase in atrial fibrillation over ten years. After the menopause, levothyroxine raises the risk of low bone density and fracture, particularly where the TSH is undetectable.

And it is common. In one study 36 per cent of patients over 65 taking levothyroxine had a subnormal TSH. More than one in three. Many of them felt perfectly well, as our headmistress does.

So the answer to her daughter is the opposite of what she expected. The dose is not too little. At a TSH of 0.06, it is more than she needs. The guideline the tool follows asks us to avoid subnormal values, and below 0.1 in particular, in older people and after the menopause.

4. When the dose does not seem to work

The opposite problem comes to every thyroid clinic: a patient on a large dose whose TSH stays high. The tool lists the causes in order, starting with adherence and timing. Two of them have studies worth knowing.

Helicobacter pylori. It sits in the tool’s list beside atrophic gastritis, coeliac disease and bariatric surgery. Treating Helicobacter took the mean TSH from 30.5 to 4.2 mIU/L in patients considered unresponsive to high doses. That is why the tool asks you to look for gastrointestinal disease, and to recheck the dose after it is treated.

The weekly dose. Some patients will not take a daily tablet, however it is explained. For them the tool offers seven times the daily dose once a week, supervised. The weekly regimen was tested in a randomised crossover of 12 patients. The pre-dose TSH was higher, 6.61 against 3.92, and the peak free T4 was 2.71 against 1.59 ng/dL, with no change in tissue markers and no demonstrable toxicity.

“So why not give everyone a weekly tablet?” Because 12 patients is a small trial, and the TSH just before each dose ran higher. It is a way of treating a patient who would otherwise go untreated, not a routine alternative. The tool also says where it is not used: in pregnancy, ischaemic heart disease or atrial fibrillation.

Back to Kundapura

Our headmistress was not under-replaced. She was over-replaced, and had been for some time, without a symptom. Her dose was brought down, and her TSH was rechecked at steady state, aiming for the range the tool prints for her age. Her daughter’s question was a good one. It simply had the opposite answer.

Try this

A 45-year-old man takes 200 mcg a day. His TSH is 28 mIU/L. He says he never misses a dose. What do you check, and in what order, before you raise the dose again? Illustrative case.

Answer. Take the tool’s order. Adherence first: it is the commonest cause, and the only one a larger prescription makes worse. Then timing: is it taken with breakfast, or with tea or coffee? Then other drugs, calcium and iron above all. Then gastrointestinal disease: Helicobacter, atrophic gastritis, coeliac disease. If he truly cannot take a daily tablet, a supervised weekly dose is the last step, not the first.

Take-home points

• Extra fat adds weight to the dose calculation without adding capacity to use the hormone; compare the lean body mass answer.
• The normal TSH rises with age; aim higher in the very old, as the tool does.
• A TSH below 0.1 in a patient over 65 is harm, even when she feels well.
• Before raising a dose that is not working, check adherence, timing, other drugs and the gut.
• A supervised weekly dose is for the patient who would otherwise go untreated, not a routine choice.

Declaration: the author built and maintains the tool. It is free to use. The figures in this article are those the tool carried in its Prescribing and Evidence tabs on 4 November 2026, each from the source the tool names. The illustrative cases follow the tool’s rules; check the tool’s result for a real patient.

References

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