According to the Physicians Desk Reference, the vast majority of pills (vitamins, etc. - really anything) are only absorbed around 20-25% So you are wasting your money with them, frankly - you absorb only a portion (and that can be just the coating & filler, which contains a lot of wax and even shellac so the pill pops out of the mold during manufacture). That's why, with 75% of the population taking supplements, we still have epidemic rates of disease & deficiencies. Our food is deficient too - fruits/vegetables are down as much as 40% in nutrients.
I have friends whose nails have improved dramatically after they switched to liquid supplementation (which the American Medical Association says is best) - you don't want a pre-mixed liquid as it degrades over time. My friends, colleagues and I all take a patented and totally comprehensive powder which you mix with water just before taking it - with its high absorption rate, we get the best value AND have the nutrition working at the cellular level in about 20 minutes. Don't take a particular vitamin or mineral - nature is designed so that all vitamins, minerals, amino acids, phytonutrients, etc. work in partnership with each other. Isolating one or two elements does not work, and it can get people in trouble. Megadosing has been condemned by all reputable authorities, but people still do it frequently.
You could have a thyroid problem but since you aren't getting enough good nutrition (none of us is, according to the AMA), you won't really know until you supplement properly.