Vitamin D Deficiency and Thyroid Disease
Theodore C. Friedman, M.D., Ph.D.
Vitamin D is an important vitamin that not only regulates calcium, but also has many other beneficial actions. Not many endocrinologists realize this, but several articles published over 20 years ago showed that patients with hypothyroidism have low levels of vitamin D.
This may lead to some of the bone problems related to hypothyroidism. It was thought that one of two mechanisms may explain the low levels of vitamin D in patients with hypothyroidism, 1) the low levels of vitamin D may be due to poor absorption of vitamin D from the intestine or 2) the body may not activate vitamin D properly.
Other articles have demonstrated that patients with Graves disease also have low levels of Vitamin D. Importantly, both vitamin D and thyroid hormone bind to similar receptors called steroid hormone receptors. A different gene in the Vitamin D receptor was shown to predispose people to autoimmune thyroid disease including Graves disease and Hashimotos thyroiditis. For these reasons, it is important for patients with thyroid problems to understand how the vitamin D system works.
Sources of Vitamin D
Vitamin D is really two different compounds, ergocalciferol (vitamin D2), found mainly in plants and cholecalciferol (vitamin D3), found mainly in animals. Both of these hormones are collectively referred to as vitamin D, and they can either be obtained in two ways. One is by exposure of the skin to the ultraviolet (UV) rays of sunlight or also from dietary intake.
Vitamin D is found naturally in fish (such as salmon and sardines) and fish oils, eggs and cod liver oil. However most Vitamin D is obtained from foods fortified with Vitamin D, especially milk and orange juice. Interestingly, as breast feeding has become more popular, the incidence of Vitamin D deficiency has increased as less fortified milk is consumed.
Vitamin D deficiency may also occur in patients with malabsorption from their intestine, such as in the autoimmune disease called Celiac Disease, which occurs frequently in patients with thyroid problems. Multivitamins also contain Vitamin D, as does some calcium supplements like Oscal-D and Citracal plus D..
Different Forms of Vitamin D and How To Diagnose Vitamin D
Vitamin D itself is inactive and needs to get converted to the liver to 25-hydroxy vitamin D (25-OH vitamin D) and then in the kidney to 1, 25-hydroxy vitamin D. It is only the 1, 25-OH vitamin D which is biologically active. This form of vitamin D acts to allow for absorption of calcium from the intestinal tract. Therefore, patients with low vitamin D levels will have low calcium and in severe cases get rickets (in children) or osteomalacia (in adults) which is when the bone bows out and is poorly formed. In mild cases of vitamin D deficiency, osteoporosis occurs.
The conversion from the 25-OH vitamin D to the 1, 25-OH vitamin D that occurs in the kidney is catalyzed by parathyroid hormone, also called PTH. Therefore, patients with low vitamin D levels will have relatively high PTH levels along with low calcium levels. This is similar to patients with primary hypothyroidism having elevated TSH levels while having normal thyroid hormone levels. Additionally, the 25-OH vitamin D form which is the storage form and is much more abundant that the 1, 25-OH vitamin D form which, although is active, is less abundant. Therefore, in states of vitamin D deficiency, low levels of 25-OH vitamin D are found, but the 1, 25-OH vitamin D levels are either normal or actually slightly high. They are slightly high because the excess PTH that is stimulated by the low 25-OH vitamin D levels stimulates the conversion up to 25-OH vitamin D to the 1, 25-OH vitamin D. Thus, patients that are vitamin D deficient usually have a low 25-OH vitamin D level, a high PTH level, a low normal calcium, and a normal or an elevated 1, 25-OH vitamin D level.
Dr. Friedman usually recommends measuring PTH, calcium, and 25-OH vitamin D to determine if a patient does have vitamin D deficiency. The 25-OH vitamin D assay has a normal range of approximately 20-60 ng/dL. However, this range may be too low for many patients. Additionally, the assay may not be that good at measuring the low levels of vitamin D. In general, Dr. Friedman would recommend treatment of patients that have a 25-OH vitamin D of less than 30 ng/dL, but these patients should have a PTH in the high normal range. Optimal levels of 25-OH Vitamin D for patients with thyroid diseases are probably 35-60 ng/dL.