The effects of COVID-19 infection on the thyroid gland can remain even after one year, according to a recent study.

Researchers at the University of Milan in Italy found that severe COVID-19 disease impacts thyroid function through a variety of mechanisms. They tracked individuals with thyroid dysfunction associated with COVID-19 illness for a year in order to better characterise such thyroid involvement and track its progression over time.

During moderate-to-severe COVID-19 disease the occurrence of thyroiditis -- inflammation of the thyroid gland -- plays an important role in thyroid dysfunction, the researchers said. This is in addition to other well-known mechanisms mainly acting on the hypothalamus-pituitary-thyroid axis, they said.

The study, presented during the 24th European Congress of Endocrinology in Milan, found that hormone imbalance is usually mild but increases in severe cases of COVID-19. "There is a clear link between thyroid dysfunction and COVID-19 disease," said Ilaria Muller from the University of Milan.

"Knowing that thyroid hormones correlate with the disease severity is important, and the fact that the thyroid gland seems directly involved in SARS-CoV-2 viral infection needs to be taken into account," Muller said. The thyroid function is crucial to the human body's metabolism, growth, and development.

By continuously releasing a stable amount of thyroid hormones into the bloodstream, it aids in the regulation of numerous body functions. The thyroid gland generates extra hormones when the body needs more energy in particular situations, such as when it is growing, cold, or during pregnancy.

The study looked at more than 100 patients admitted to the hospital with severe COVID-19, analysing their thyroid-stimulating hormone (TSH) and other indicators. Thyroiditis occurred frequently in the COVID-19 patient population and the thyroid function, as well as inflammatory indicators, returned to normal in nearly all instances shortly after the end of their COVID-19 illness, the researchers said.

However, after 12 months thyroiditis regions remained visible at thyroid ultrasound in half of the individuals, even if reduced in size. The thyroid uptake of technetium or iodine, an indicator of thyroid function, was still reduced in four out of six individuals at nine months, although it had mostly recovered after 12 months.

The long-term clinical consequences, if any, are unknown, according to the researchers.