The thyroid is a butterfly-shaped gland in the throat. There are no evidence-based, peer reviewed studies that suggest vitamin K plays a role in regulating its function. There are many different reasons for thyroid issues so you will have to work with your doctor to determine if something is wrong and what to do about it. These problems are best approached by conventional medicine.
The discovery of vitamin K stemmed from work Danish researchers Henrik Dam performed on the cholesterol metabolism in chicks between 1928 and 1930. He observed that chicks receiving a cholesterol-free chicken feed began to experience hemorrhages under the skin muscles and other organs. Investigation showed that this feed also lacked another substance that was responsible for the clotting of blood. In 1935, scientists characterized it as a new fat-soluble vitamin and named it vitamin K, for "koagulation," the German word for clotting.
Vitamin K Biochemistry
Vitamin K is a co-factor for the enzyme that converts the amino acid glutamic acid into gamma-carboxyglutamic acid. Without vitamin K, this reaction would not occur efficiently, and it is critical to the clotting process, because several of the proteins in which it occurs are clotting factors. If this reaction did not occur, those proteins could not bind calcium, which is essential for clotting.
The thyroid gland makes two hormones, thyroxine and triiodothyronine. Together, these hormones regulate how efficiently cells metabolize glucose. If levels of these hormones are low, cells cannot turn glucose into energy efficiently. A thyroid function test involves testing the levels of thyroxine, triiodothyronine and thyroid-stimulating hormone, which is made by the pituitary gland.
While vitamin K does not affect the thyroid, thyroid function affects the efficiency of blood clotting. Most of the research in this area is from a small number of papers published in the 1970s. For example, a study appearing in the 1976 issue of the journal "Thrombosis and Haemostasis" showed that when the thyroid function of rats is slowed, the metabolic rate of cells producing clotting factors also slows. A 2008 article in the "International Journal of Immunopathology and Pharmacology" showed that thyroid activity affected levels of particular clotting factors, but otherwise added no new information.