Two People, the Same Thyroid Number, Two Different Diseases

Two women sat in my clinic in the same week with a TSH of 8.4. Same number, same symptoms, same referral letter.

One had an immune system attacking her thyroid, a condition that would progress, would need lifelong monitoring, and carried a raised risk of three other autoimmune conditions I now had to screen her for.

The other had a thyroid that had been temporarily knocked out by a viral illness six weeks earlier and would recover on its own within a few months. Putting her on lifelong levothyroxine would have been a mistake she carried for decades.

Nothing on the standard panel distinguished them. An underactive thyroid is a finding, not a diagnosis, and the finding is where most workups stop.

There are two questions that should be asked next. Neither is on a routine request form.

Question one: is the immune system involved

This is answered by thyroid peroxidase antibodies, usually with thyroglobulin antibodies alongside.

If they are positive, you are looking at autoimmune thyroiditis, and the whole shape of the illness changes. It is progressive, so a mildly raised TSH today is likely to be a clearly raised one in a few years. It needs monitoring rather than a single reassuring visit. It travels with other autoimmune disease, particularly celiac, type 1 diabetes, vitiligo, and pernicious anaemia, which means screening beyond the thyroid. In pregnancy it carries its own risks and lowers the threshold for treatment considerably.

If they are negative, none of that follows automatically, and the question becomes what else did this.

I hold antibody results with one caveat. A minority of genuinely autoimmune thyroid disease is antibody negative, so a negative result narrows the field rather than closing it. Ultrasound appearance and how the picture behaves over time both add information.

Question two: is it even coming from the thyroid

This is the one that gets missed, and it is the reason I dislike TSH-only screening.

The pituitary is what produces TSH. If the pituitary is the organ that is failing, it does not raise TSH when thyroid hormone falls, because raising TSH is precisely the function it has lost. So the result comes back with a low free T4 and a TSH that reads normal, or even low.

Screen with TSH alone and that patient is reported as fine. This is called central hypothyroidism, and the published guidance describes it as the most important false negative of the reflex TSH strategy. It is uncommon, and it is not rare enough to ignore, particularly with a history of pituitary surgery, head injury, cranial radiation, postpartum haemorrhage, or any other pituitary hormone already known to be low.

Two things about it change management completely.

Cortisol has to be assessed first. Starting levothyroxine in someone who also has an undiagnosed cortisol deficiency can precipitate an adrenal crisis, because thyroid hormone increases the rate at which the body clears cortisol. If the pituitary has failed at one axis it may well have failed at another. Guidance is explicit that treatment begins once a concomitant cortisol deficit has been excluded, and where one exists, glucocorticoid replacement comes first.

TSH becomes useless for monitoring. The signal you would normally follow is the broken part. Dosing is guided by free T4 instead, rechecked six to eight weeks after starting and targeted above the middle of the reference range. A patient with central hypothyroidism whose dose is being adjusted by TSH is being managed by a number that cannot respond.

The thyroid that is simply tired

When antibodies are negative and the pituitary is working, there is a real list to go through.

Anything that removed or damaged thyroid tissue: surgery, radioiodine, external beam radiation to the neck.

Medications, which are more common than people expect. Amiodarone, lithium, interferon, tyrosine kinase inhibitors, and the immune checkpoint inhibitors used in oncology all cause thyroid dysfunction, and the last group does so often enough that thyroid function is monitored as standard during treatment.

Iodine, in both directions. Deficiency where intake is genuinely low, and excess from high dose supplements, kelp products, or contrast media.

And the transient thyroiditis group, which is the one that changes the answer most. Subacute thyroiditis after a viral illness, classically with a painful thyroid and a phase of overactivity before the underactive phase. Postpartum thyroiditis in the months after delivery. Both frequently resolve.

That distinction matters more than any other on this page. A person with transient thyroiditis put on levothyroxine and never rechecked is treated for life for something that fixed itself in four months. It happens, and it is entirely preventable by retesting off treatment once the picture is stable.

If any of this sounds like your experience, you can book a virtual consultation with Dr. Sater to review your history and testing in full.

The panel to ask for

If you have been told your thyroid is underactive and nothing further was checked, the panel to request is TSH with free T4, free T3, TPO antibodies, and thyroglobulin antibodies, together with a proper history of medications, recent illness, recent delivery, and any neck surgery or radiation.

The number tells you the thyroid is underperforming. It does not tell you why, whether it will progress, whether it will recover, or whether the problem is even in your neck.

Those are four different answers, and they lead to four different plans.

Source: The diagnosis and management of central hypothyroidism, Endocrine Connections 2019

This article is educational and is not individual medical advice. Thyroid results should be interpreted by a physician who knows your full history.