Abnormal Thyroid Function with Statins

Recently it has been observed that many people on statins have been referred for special radioiodine studies because they have been found to have low to no thyroid stimulating hormone (TSH) in their blood tests.

Usually a low serum TSH suggests hyperthyroidism (overactive thyroid function) but these low TSH patients often had no signs of hyperthyroidism and had normal to high-normal thyroid hormone levels when they were measured prior to referral. Subsequent radioiodine uptakes and scans on these patients were normal.

This chance observation by one of the authors of this paper resulted in a review of more than 300 low TSH patients on statins who had been referred to the Nuclear Medicine Service at Presbyterian Hospital of Dallas over a 4-year period to determine whether statin exposure affected the outcome of the TSH test.

The TSH test is based upon the pituitary gland's ability to produce this substance when the output of the thyroid gland is low for any reason. Ordinarily this test is a very reliable indicator of thyroid function and a low TSH test means high thyroid function but the use of statins clearly was altering this relationship.

Participants included 29 patients taking a statin medication and 278 who were not taking a statin. Sixteen of the 29 patients who were taking a statin (55%) had normal radioiodine uptakes and scans despite their low serum TSH. The remaining 13 low TSH patients who were taking a statin (45%) had abnormal uptakes and scans, most commonly showing diffuse thyroid hyperplasia ( large increase in number of cells in the thyroid ) with increased radioiodine uptake. In contrast, the vast majority of the 278 patients not taking a statin had abnormal uptakes and scans.

Females outnumbered males by approximately 3:1 in this study and females were on average 6 years younger than their male counterparts. Of the 29 patients who reported using a statin, 16 were taking atorvastatin, 6 simvastatin, 5 pravastatin, 1 fluvastatin, and 1 lovastatin, reflecting the common observation that all of the different brands of reductase inhibitors have the same general side effect profile.

The results of this study show that statins may falsely lower the serum TSH without altering thyroid function, the condition observed in the 16 of the 29 statin users described above who had normal scans and uptakes on follow-up examinations. In so doing they mimic a clinical condition known as pseudohyperthyroidism or falsely overactive thyroid.

Alternatively there is another explanation. Statins may improve thyroid function in patients with hyperthyroidism. The same 16 out of 29 statin users might all be hyperthyroid cases who were responsive to the anti-inflammatory action of the statins, suggesting that, if all 308 patients enrolled in this study with low TSH had been using statins,  some 55% would have shown normal thyroid function on follow-up testing because the inflammatory activity underlying the condition would have been suppressed by the anti-inflammatory effect of the statins.

Bottom line is that strange things may happen to thyroid tests in those on statins but they are not necessarily all bad. This does reflect the disturbing fact that some 20 years after marketing of statins first began we are still finding new mechanisms by which they can affect our bodies.

Duane Graveline MD MPH
Former USAF Flight Surgeon
Former NASA Astronaut
Retired Family Doctor

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