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From the information that you have provided, it appears that you have developed gradual onset of a "deep groove" in the nails of your middle fingers on both hands. You stated that, initially, the right hand seemed to get better, but, now, again has the deep groove similar to the left hand. One question, does the deep groove look similar to a horizontal line across the nail from one side of the finger to the other? OR, does the deep groove look similar to a vertical line that runs from the tip of the nail to the cuticle area? Or, perhaps, neither?
I assume that you have never had this before, it appears to be somewhat of a new thing, present for less than 6 months, and you have never previously had a similar problem, correct?
Believe it or not, vitamins and nutrients are essential for normal nail growth, and the appearance of your nails is a good indicator of your physiological health (meaning what is going on internally in your body).
The presence of "deep grooves" or lines in your nails can represent a nutrient or vitamin deficiency. The presence of horizontal lines in your nails, can also just be part of the normal aging process, but, in your case, you have vertical lines, and it seems as though the changes in your nails are quite substantial and significant and wouldn't be best described as a normal aging process. It wouldn't be expected that you would develop deep grooves in your nails as a normal process of aging, the lines associated with normal aging are more subtle. Are there any changes to your toe nails? This may be interesting for you to look into after I provide you some information in this regard because what you may find is that there are also changes on your toe nails similar to the hands.
Okay, getting down to the business at hand, several possibilities exist. Adequate intake of vitamins and nutrients is essential for normal nail growth. For example, calcium is necessary for proper nail growth; vitamin D is also necessary for the normal growth of your nails. A deficiency of calcium and vitamin D could result in brittle nails that break very easily and/or dry nails that "peel" in layers. Similar to this, Vitamin A is also an essential nutrient for normal nail growth. Lack or deficiency of Vitamin A could result in horizontal and/or vertical ridges (or grooves) in your nails, and can cause "fragile" or brittle nails that break easily. This could be one explanation for your question. The best source of Vitamin D is exposure to sunlight. Your skin absorbs Vitamin D just by standing in the sun light. Sometimes, if you live in an area where the seasons change, during the winter, you can be deficient in Vitamin D just because you are not exposed to the sun like you would be in the summer.
Another possibility, Vitamin B deficiency, namely B12, can cause dry nails, curved nail ends, and darkened appearance of nails. Vitamin B12 deficiency is also known to cause horizontal and/or vertical ridging (or grooves) similar to deficiency of Vitamin A.
Another important cause to consider is iron deficiency anemia, which can lead to a brittle and "ridged" texture to your nails. It could also cause your nails to look flat or concave (curved opposite of what is expected), and nails may become brittle, thin and pale.
In all cases, any changes in the structure of your nails, whether it is the deep grooves that you describe or the shape or appearance of your nails, it usually suggests a vitamin or nutrient deficiency. Have you ever been diagnosed with iron deficiency anemia? Have you ever had to supplement iron daily? Iron deficiency anemia can also cause fatigue, weakness and a general feeling of not being well. You doctor can order lab tests that can determine if you are anemic and, if you are found to be anemic, it can also be determined if the anemia is caused by iron deficiency or another cause.
It is always important to take a multi-vitamin daily. Given your age, probably recently post-menopausal, you may be having changes in your nails related to possible calcium or vitamin D deficiency, which becomes very common in post-menopausal women. If you do not already take a daily multi-vitamin, you should talk with your doctor about this. Usually, for post-menopausal women, it is also important to supplement calcium and Vitamin D, which can also be discussed with your doctor. Vitamin B complex is available over the counter and can be supplemented daily. A vitamin B deficiency can also cause fatigue, weakness, and neurological symptoms like numbness and tingling in your hands and feet.
Is there anything that I can clarify for you in this regard? Taking a multivitamin is important to your overall health and can also help with your nails.
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