Fat cells produce estrogen. Estrogen protects hair follicles on your head from the destructive effects of androgens. If you lose a lot of weight, especially if you lose it quickly, your estrogen levels can fall and that can cause hair loss. It has happened to me and many others on 3fc.
The best thing I found for my thinning hair Nizoral shampoo. When I switched to Nizoral, my hair started to come back pretty quickly!
Here's a bit from wikipedia on ketoconazole, the active ingredient:
"Limited clinical studies suggest that ketoconazole shampoo used either alone[19][20] or in combination with other treatments[21] may be useful in reducing hair loss."
Now, that doesn't sound very dramatic, but my experience was!