---
title: Korea's cancer registry told a journal that a study of rising thyroid cancer deaths had not measured mortality
publication: Medical Insights Korea
section: news
category: Thyroid
canonical_url: https://medicalinsightskorea.com/news/thyroid-mortality-versus-case-fatality-korea
author: James Choi
published: 2026-09-02T10:40:00+09:00
modified: 2026-09-02T10:40:00+09:00
content_type: reporting
license: Quotation permitted with attribution to Medical Insights Korea and a link to the canonical URL.
---

# Korea's cancer registry told a journal that a study of rising thyroid cancer deaths had not measured mortality

The paper reported thyroid cancer-specific mortality rising from 0.76 to 2.70 per 1,000 person-years. The national registry's reply says that is a case fatality rate, and that population mortality fell from 1.06 to 0.80 per 100,000 over almost the same period.

## Answer

A 2024 study of 434,228 Korean patients reported thyroid cancer-specific mortality falling to 0.76 per 1,000 person-years in 2013 and rising to 2.70 by 2018, and the Korea Central Cancer Registry responded in print that this is a case fatality rate rather than a mortality rate, because it counts deaths among diagnosed patients rather than in the general population, where the age-standardised thyroid cancer mortality rate fell from 1.06 per 100,000 in 2012 to 0.80 in 2018.

## Key takeaways

- A nationwide cohort study of 434,228 Korean thyroid cancer patients reported thyroid cancer-specific mortality falling from 1.94 per 1,000 person-years in 2005 to 0.76 in 2013, then rising to 2.70 by 2018.
- The Korea Central Cancer Registry replied in the same journal that this measure counts deaths among diagnosed patients and is therefore a case fatality rate; a mortality rate counts deaths in the general population.
- By the registry's figures the age-standardised thyroid cancer mortality rate in Korea fell from 1.06 per 100,000 in 2012 to 0.80 in 2018, while the case fatality rate in the study rose.
- The registry offered two further objections: a rate computed with person-years assumes constant risk when the risk of cancer death is highest in the first year, and shorter follow-up for patients diagnosed later can inflate the figure.
- The registry's own table shows annual thyroid cancer deaths in Korea remaining between 330 and 394 throughout 2005 to 2018, a period in which annual new diagnoses ranged from 12,833 to 44,798.

## Full article

In September 2024 the International Journal of Surgery published a study with an arresting conclusion: after years of decline, thyroid cancer deaths in Korea had started rising again among patients diagnosed since 2015 — at the same time as diagnoses were falling. It read as a warning that the campaign against overdiagnosis had gone too far.

Six months later the Korea Central Cancer Registry replied in the same journal. Its objection was not to the arithmetic. It was to the name.

### What the study reported

Researchers used the National Health Insurance Service database to follow 434,228 people diagnosed with thyroid cancer, with a median follow-up of 7.4 years, and calculated what they called the thyroid cancer-specific mortality rate by year of diagnosis.

**Thyroid cancer-specific rate per 1,000 person-years, as reported in the 2024 study**

| Year of diagnosis | Rate |
| --- | --- |
| 2005 | 1.94 |
| 2013 | 0.76 |
| 2018 | 2.70 |

Over the same span the standardised incidence rate went from 91.9 per 100,000 in 2012 down to 50.6 in 2015. Diagnoses nearly halved while this figure more than tripled, and the authors read that as evidence that appropriate diagnosis of high-risk disease matters alongside the effort to reduce overdiagnosis.

### The registry's first objection: this is not mortality

Statisticians at the Korea Central Cancer Registry, part of the National Cancer Center, wrote that the analysis counts deaths among people already diagnosed with thyroid cancer, which is a case fatality rate. A mortality rate counts deaths in the general population over a period.

> **Why the distinction is not pedantry**  
> Mortality measures what a disease costs a population. Case fatality measures how dangerous the disease is to the people who have it. If more people are diagnosed, the same number of deaths produces a lower case fatality rate and an unchanged mortality rate. If fewer are diagnosed, case fatality rises with no additional person dying.

The registry added that case fatality is a reasonable measure for an acute illness, where progression is fast and death follows soon, and a misleading one for a cancer where the risk of dying of something else grows with every year of follow-up.

### The second objection: the rate's shape

Expressing the figure per person-year assumes the risk is constant over time. The registry points out that it is not: the risk of death after a cancer diagnosis is highest in the first year and falls thereafter, becoming minimal after about five. Patients diagnosed in 2018 had been followed for a fraction of the time of those diagnosed in 2013, so a larger share of their observed follow-up sits in the high-risk window — which can inflate the later figures without any change in the disease.

### The third objection: who is in the cohort changed

As screening of low-risk people fell away, the registry argues, the remaining diagnosed population contained a higher proportion of advanced disease. A case fatality rate calculated over a sicker cohort rises even if every individual's prognosis is unchanged. Its published table shows the shift.

**Korean thyroid cancer by stage and population mortality, from the registry's reply**

| Year | New cases | Localised | Regional/metastatic | Deaths | Mortality per 100,000 |
| --- | --- | --- | --- | --- | --- |
| 2005 | 12,833 | 50.4% | 49.6% | 330 | 1.28 |
| 2012 | 44,798 | 46.5% | 53.5% | 376 | 1.06 |
| 2013 | 43,124 | 44.5% | 55.5% | 394 | 1.08 |
| 2015 | 25,576 | 42.3% | 57.7% | 341 | 0.85 |
| 2016 | 26,679 | 42.0% | 58.0% | 346 | 0.82 |
| 2018 | 29,070 | 47.2% | 52.8% | 372 | 0.80 |

The localised share falls from 50.4 percent in 2005 to 42.0 percent in 2016 as the registry describes — and then moves back to 47.2 percent in 2018, which their own table records and their argument does not address. Read strictly, the case-mix explanation fits the years to 2016 better than it fits the last year in the series.

### The column that settles what the population experienced

The most direct answer in that table is not a rate at all. It is the raw count of deaths.

**330 to 394** — the range of annual thyroid cancer deaths in Korea across 2005 to 2018, a period in which annual new diagnoses ranged from 12,833 to 44,798  
_Source: Jung KW et al., International Journal of Surgery, 2025_

Diagnoses tripled and then halved. Deaths stayed inside a band of sixty-four. The age-standardised mortality rate fell from 1.28 per 100,000 in 2005 to 0.80 in 2018 — the opposite direction to the figure that made the original study's headline, because it is a different figure.

### What this exchange does not resolve

1. The study's authors published a response in the same journal in November 2025, titled "Understanding thyroid cancer-specific mortality metrics: a response to recent commentary". It is behind a subscription and we have not read it, so nothing here should be taken as representing their answer.
2. The registry's objection is to naming and interpretation, not to the underlying counts. Whether case fatality among diagnosed Korean patients genuinely rose after 2013 is a separate question, and the confounders the registry names — follow-up length and case mix — would need to be handled before it could be answered.
3. Falling population mortality is not by itself proof that reduced screening caused no harm. A small number of avoidable deaths could sit inside a total that stays between 330 and 394 without moving it detectably.
4. The registry's case-mix argument fits the stage distribution through 2016 and not the 2018 figure in its own table.

> **Why this matters beyond one paper**  
> Both figures in this exchange are real and both were correctly calculated. Only one of them answers the question a reader assumes is being answered when they see the word mortality. A statistic that is accurate and misnamed travels further than one that is wrong, because nothing about it fails a check.

## Sources

- Kim KJ, Choi J, Park SK, Park YJ, Kim SG. Thyroid cancer-specific mortality during 2005-2018 in Korea, aftermath of the overdiagnosis issue: a nationwide population-based cohort study. International Journal of Surgery 2024;110(9):5489-5495. doi:10.1097/js9.0000000000001767. PMID:38874484. https://doi.org/10.1097/js9.0000000000001767
- Jung KW, Park EH, Kang MJ. Clarifying the misinterpretation of case-fatality rate as mortality in thyroid cancer studies. International Journal of Surgery 2025;111(6):4103-4104. doi:10.1097/js9.0000000000002133. PMID:40085754. https://doi.org/10.1097/js9.0000000000002133
- Kim KJ, Choi J, Park SK, Park YJ, Kim SG. Understanding thyroid cancer-specific mortality metrics: a response to recent commentary. International Journal of Surgery 2026;112(2):5583-5584. doi:10.1097/js9.0000000000003769. PMID:41208606. https://doi.org/10.1097/js9.0000000000003769
- Yoon YJ, Seo JM, Kim JS, Choi YM, Kim WG. Long-Term Trends in Thyroid Cancer Incidence and Mortality in Korea: Analysis of Korean National Data. Endocrinology and Metabolism 2026. doi:10.3803/enm.2025.2807. PMID:42324644. https://doi.org/10.3803/enm.2025.2807

## Citation

James Choi (2026). "Korea's cancer registry told a journal that a study of rising thyroid cancer deaths had not measured mortality". Medical Insights Korea. https://medicalinsightskorea.com/news/thyroid-mortality-versus-case-fatality-korea

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