Every study to date, assessing the effects of either Green Tea supplementation or topical application on hair loss, has produced positive results.
     Green Tea has many identified mechanisms which could account for these reliable hair growth stimulation effects. Among these identified to date, mechanisms are 5 alpha reductase inhibition, and Inhibition of Tumor Necrosis Factor (TNF). It is also regarded as a Metformin mimetic, (AMPK activator) and an inhibitor of the MTOR pathway, conferring potent anti-aging benefits as well.  A new study demonstrates that Epigallocatechin Gallate, the most studied component of Green Tea, may stimulate hair growth on a genetic level as well.  Epigallocatechin gallate (EGCG), also known as epigallocatechin-3-gallate, is the ester of epigallocatechin and gallic acid, and is a type of catechin. EGCG, is the most abundant catechin in Green Tea.
     A recent study (abstract below), demonstrates EGCG, among other identified mechanisms, has been shown to stimulate hair growth in mammal models via activation of the Sonic Hedgehog pathway.
     Sonic hedgehog (SHH) is the best studied ligand of the vertebrate pathway. The SHH gene provides instructions for making a protein called Sonic Hedgehog. It is required for transition of the hair follicle from the resting to the growth phase. Recent research has shown that activation of Sonic Hedgehog signaling   produces hair growth, even in conditions, such as wounding and long standing peri-follicular inflammation ( as seen in slick bald for many years, men) were hair growth was considered not at all possible;   “Our results show that stimulating fibroblasts through the sonic hedgehog pathway can trigger hair growth not previously seen in wound healing.” Mayumi Ito, Ph.D.
     Dr. Ito is now focusing on identifying compounds that   activate sonic hedgehog signaling, which have implications for both halting the progression of, and reversal of hormonal and other forms of hair loss.       As it turns out, Green Tea has now been identified as an activator of the Sonic Hedgehog pathway, and as such, produces hair growth effects. 
Front Pharmacol. 2018 Jun 26;9:674. Epigallocatechin-3-Gallate Promotes the Growth of Mink Hair Follicles Through Sonic Hedgehog and Protein Kinase B Signaling Pathways.AbstractBackground: Hair follicles play an essential role in the growth of hair. Epigallocatechin-3-gallate (EGCG), a catechin polyphenol in green tea, has various bioactivities. The present study aims to evaluate the effect of EGCG on the growth of mink hair follicles and investigate the possible molecular mechanisms. Methods: The length of hair follicles was recorded up to 6 days in presence of 0.1-5 μM EGCG. Primary dermal papilla cells (DPCs) and outer root sheath cells (ORSCs) were treated with 0.25-4 μM EGCG, and their growth was evaluated by MTT assay and cell cycle detection. The levels of key molecules in sonic hedgehog (Shh) and protein kinase B (AKT) signaling pathways were further assessed by quantitative real-time PCR, western blot and immunofluorescence. To determine the involvement of Shh and AKT pathways in EGCG-mediated growth-promotion of ORSCs and DPCs, Shh pathway inhibitors cyclopamine and GANT61 or AKT pathway inhibitor LY294002 were employed, and then cell proliferation and cell cycle were analyzed. Results: Data from ex vivo culture showed that, in presence of 0.5-2.5 μM EGCG, the growth of mink hair follicles was promoted. In vitro, the proliferation of DPCs and ORSCs was enhanced by 0.5-4 μM EGCG treatment. More cells entered S phase upon treatment of EGCG, accompanied with upregulation of cyclin D1 and cyclin E1. Furthermore, when exposed to EGCG, the Shh and AKT signaling pathways were activated in both hair follicles and primary DPCs and ORSCs. Inhibiting either of these two pathways partly reversed the effect of EGCG on proliferation and cell cycle of DPCs and ORSCs. Conclusion: EGCG promotes the growth of mink hair follicles at concentrations of 0.5-2.5 μM. This growth-promoting effect of EGCG is associated with the increased proliferation of DPCs and ORSCs through activating Shh and AKT signaling pathways.
            The hair follicle concentration of EGCG used in this study should be achievable through a moderate to high level of oral  supplementation.
     Here is just a sample of some published studies assessing the effects of Green Tea on mechanisms related to hair growth: 
Human hair growth enhancement in vitro by green tea epigallocatechin-3-gallate (EGCG).

The effects of tea polyphenolic compounds on hair loss among rodents

Epigallocatechin Gallate-Mediated Alteration of the MicroRNA Expression Profile in 5α-Dihydrotestosterone-Treated Human Dermal Papilla Cells

Green tea in dermatology

      We have recommended high dose Green Tea supplementation for many years, as part of any hair loss regime.

     The specific Green Tea Extract we recommend is Mega Green Tea Extract, which has by far the highest dose of EGCG on the market.
MEGA GREEN TEA EXTRACT Green Tea Extract has been shown in several studies to be a potent inhibitor of 5-alpha reductase type I,the isozyme that converts testosterone into DHT in skin. Propecia (Proscar) inhibits 5-alpha-reductase type II, which is located primarily in the prostate gland, and is responsible for the amount of DHT in serum circulation. It is also thought to have use as an androgen receptor blocker. Additionally Mega Green Tea Extract has been shown to neutralize the inflammatory cytokines associated with hair loss and to inhibit aromatase, often ameliorating the side effects associated with Propecia, allowing for its continued usage. Combining high potency Green Tea Extract with Propecia (Proscar) should increase its effectiveness, and provide numerous additional health benefits, including a reduction in bodyfat. A recent study also demonstrated that EGCG, a component of Green Tea, also promotes hair growth through the Sonic Hedgehog signaling pathway

One 725 mg capsule,of Mega Green Tea Extract 98% Extract, provides 326.25mg EGCG, which is the largest dose of EGCG commercially available in any Green Tea Extract supplement. Mega Green Tea Extract, or any Green Tea Extract supplement, is far better absorbed by taking it with either a few drops of fresh lemon juice in water or a vitamin C tablet. Green Tea Extract would be a highly recommended, effective, and cost effective addition, to any existing hair loss treatment regime.