Step 1 - Rule out medical reasons.
If your hair is excessively shedding - for example, if a noticeable amount of hair strands collect on the carpet despite weekly hoovering , then consider that as a sign to do something. Rule out thyroid issues, low iron (ferritin) and coeliac gluten intolerance by booking those three blood tests at your local GP practice. Go on, do it now. If any of these tests come back requiring action then your hair will be suffering and no amount of loving care will improve it, without medical help.
Remember that your test results being 'in range' does not mean that they are in optimum range for you. For example, the goal posts for thyroid results recently changed (meaning results that would previously have been concerning and medication-requiring are now considered 'in range').
When getting a thyroid blood test, if you are already on thyroxine medication, you need to follow certain protocol on the preceding days and on the day of the test itself, to get an accurate reading. Book a blood test first thing in the morning, as you need the blood to be taken BEFORE you take your thyroxine tablet(s) which also means no eating beforehand (and then for at least 30 minutes after you do manage to take your medication - which usually for me is as I'm walking out of the GP surgery, absolutely starving!) Also, if you take a supplement called 'Biotin' (see more about supplement suggestions below) then DO NOT take the Biotin for THREE DAYS before getting the blood test. Both Biotin and your thyroxine tablet can give falsely lower readings (i.e. indicating that your current medication levels are working 'better' than they actually are).
As a rule of thumb, you want your thyroid TSH levels to be between 0.5 and 2.0 ulU/mL (micro-international units per millilitre) for optimum thyroid function. If they are higher than this, request an increase in your thyroxine dose to see an improvement in hair condition. It really does make a difference WHEN you take your thyroid medication. It must be taken on an empty stomach, with just a couple of sips of water (definitely no caffeine and definitely no cow's milk to wash it down!) and then you need to wait 30-60 minutes before eating or drinking. So taking thyroxine as soon as you wake up, first thing in the morning, is usually the best option. You MUST NOT take any supplement for 4 hours after your thyroxine (especially not iron-containing supplements) - I always make sure that I take my iron supplement four hours apart from other supplements too. Iron really impedes the absorption of other pills. Dairy (regular milk, yoghurt, cheese etc) also inhibits absorption of meds and supplements too. Any tablets containing hormones need to be taken 4 hours or more away from your thyroxine also, so taking the Pill last thing at night may be helpful to consider, if you need to fit that into your medication schedule too! If planning a schedule yourself is too tough, try using Chat GPT and asking the AI search function to 'come up with a good schedule showing what to take when from this list:' and then type in your list of meds and supplements. The website to use is.... https://openai.com/index/chatgpt/
As a rule of thumb you want your ferritin levels to be 80-100 ug/L (micro-grams per litre) for optimum hair growth. You will only be prescribed high-dose iron (which gives you 140mg iron per 5ml teaspoon) if your ferritin levels are below 15 ug/L. So you may need to source your own iron liquid supplement, but be warned that most over-the-counter liquid iron supplements contain wheat (in the form of barley) so are unsuitable if you do turn out to be gluten-intolerant/coeliac. Liquid iron is always preferable over iron tablets, as iron tablets will cause unpleasant constipation. The slight constipation associated with taking liquid iron can be alleviated by eating 6-12 prunes (or similar) daily. If you take an iron-only supplement, make sure that you also take something containing a high dose of Vitamin C, as a lot of Vitamin C is needed for iron absorption. Acerola (made from natural cherries) vitamin C is the best to aim for, e.g. Naturelo brand on eBay.
If you discover that you are coeliac (meaning that even a crumb of gluten ingested will stop your small intestine from properly absorbing nutrients for 6 weeks to 6 months after ingesting it) or if you are gluten intolerant, or if you have some other food intolerance, then taking the above medicines and supplements won't make much difference because your gut is too inflamed or too 'leaky' to absorb the good stuff. If your coeliac (gluten) test comes back negative, then systematically try cutting out common trigger food groups one at a time until you notice what calms your tummy (less gas, less bloating, normal poo - see the 'Bristol Stool Chart'). I found the York Test postal blood test kit to be expensive, but very useful to identify my own food triggers. BUT never cut out gluten until AFTER you have been to the GP for a coeliac blood test, as a coeliac test relies on you eating gluten plentifully for the 6 weeks preceding the coeliac test!
What you can start doing immediately to improve your hair growth, is improving your gut microbiome by drinking a glass of kefir (fermented yoghurt drink available in supermarkets) every day. If lactose intolerance is suspected, take (pricey) probiotic capsules e.g. Boots 'Good Gut ' Max Strength capsules instead.
If you are diagnosed as having an underactive thyroid, it is definitely a good idea to get your 'thyroid antibodies' checked to see if your issues are cause by Hashimoto's disease (it is estimated that 90% of underactive thyroid issues in the western world are cause by Hashimoto's disease). This thyroid antibody testing service is not available on the NHS. Using a postal blood testing service such as Medichecks is a good option. If your thyroid antibodies are raised then it indicates that you need to work on a multi-pronged approach to managing your Hashimoto's disease, alongside taking your thyroxine. Measures include: actively avoiding stress, getting plenty of sleep, avoiding gluten (and possibly soya and dairy), avoiding alcohol, avoiding caffeine, avoiding excessive exercise, caring for gut health and taking thyroid-supporting supplements such as Selenium, Biotin, Glucosamine, Myo-Inositol and L-Glutamine.
For much more information about managing an underactive thyroid cause by Hashimoto's disease, I suggest reading anything and everything (online on Instagram and books) by Dr Izabella Wentz, a pharmacist in the USA who herself suffers from underactive an underactive thyroid caused by Hashimoto's disease. Rachel Hill is a UK thyroid advocate who posts regularly on Instagram as theinvsiblehypothyroidism - her advice has greatly helped me too.