The new count, released on Monday, covers samples from clinics in Stockholm, Uppsala and Västerås tested between May and September. The network had been investigating a rise in results suggesting overactive thyroids; it now says some readings may have been skewed by high levels of biotin, a vitamin found in many hair and nail supplements.
The review began with 220 samples from patients whose symptoms did not fit their test results. A laboratory recheck found interference in 41. The expanded audit has now assessed 7,240 samples, with 1,386 flagged for repeat testing, according to the Svea Diagnostic Service, a regional laboratory provider.
A small audit grows
The suspected link emerged during a follow-up to a months-long rise in abnormal thyroid readings. Clinicians had reported patients with few signs of thyroid disease receiving results that appeared to indicate an overactive gland. Online posts promoting a so-called “thyroid reset” had encouraged people to take concentrated biotin supplements, claiming the vitamin could improve energy and make routine tests more revealing.
Nia Hepomäki-Vasala, an endocrinologist advising the review, said biotin can interfere with some laboratory methods and make hormone levels appear different from their true values. “We expected a handful of affected samples. We did not expect this to account for almost one in five of the results we checked,” she said. The laboratory says the flagged readings are not all confirmed errors; repeat tests will establish which were unreliable.
We expected a handful of affected samples. We did not expect this to account for almost one in five of the results we checked.
Nia Hepomäki-Vasala, endocrinologist
The scale became clear when technicians compared the original results with repeat measurements using a different testing method. Of 412 samples examined in that second stage, 79 showed a substantial change in at least one thyroid marker. The network said the finding does not establish that every flagged patient took the supplement, but it has prompted clinics to ask about vitamins and other products before drawing new blood samples.
Clinics begin retesting
Céline Allard, a spokesperson for Sweden’s Health Products Board, said the agency had received 86 reports from clinicians since June about puzzling thyroid results. It is reviewing the wording on supplement labels and has asked sellers to add a warning that biotin can affect some blood tests. The agency has not ordered a recall and said the products remain available.
Clinics are contacting patients whose care may have relied on a flagged result. Fourteen people had started thyroid medication after tests now under review, according to the laboratory network; their clinicians are checking each case before making changes. No serious harm has been reported, and doctors have been told not to stop or alter prescribed treatment without speaking to a clinician.
The retesting is expected to cost 28 million kronor (about €2.5 million), paid initially by the regional health authorities. The first appointments began on Monday, with priority given to patients whose medication or planned treatment depends on the original result. The network expects to complete the checks by mid-December.
Allard said the agency was asking online platforms to correct posts that present supplements as a way to improve thyroid function. Hepomäki-Vasala urged patients to bring supplement bottles or a list of products to appointments. “A result that looks dramatic is not a diagnosis by itself,” she said.
