Italian researchers have discovered an increased risk of lung cancer in women using synthetic T4. The original article has been published in several medical journals and a link to the PDF article is available at Greenmed.

The article criticizes treating mild hypothyroidism, but the real criticism should be for the cavalier destruction of the thyroid gland as a treatment for hyperthyroidism. Telling patients that lifelong hypothyroidism is easily treated is not only untrue, but puts us at risk of not being able to procure sufficient replacement hormone. People who have had radioiodine ablation or thyroidectomy surgery face a lifetime of full replacement hormone.  After 6 weeks without needed replacement hormone, patients needing full replacement hormone are at risk for myxedema coma.

Problems associated with the manufacture of synthetic T4 keep escalating. And with the potential for lawsuits from patients on levothyroxine who now have lung cancer, there's not much profit in manufacturing such an inexpensive drug. 

The risk of lung cancer has not been seen in patients using natural glandular thyroid extract. 

Since levothryoxine was first synthesized and hit the market there have been critics. In one lawsuit, patients on Synthroid were reimbursed because of false advertizing that suggested Synthroid was superior to generic products. Pharmaceutical companies also insisted that T4 alone was adequate for providing sufficient converted T3. This has also been shown to not be the case.