Hypothyroidism (underactivity of the thyroid gland) is diagnosed through blood tests when cats show symptoms such as unexplained weight gain, lethargy or lack of desire to play and excessive thirst. Vetinfo supplies a list of other signs, such as dryness and dandruff in the cat's coat, lowered body temperature, constipation and lack of appetite.
According to Vetinfo, hypothyroidism in cats doesn't often occur naturally. Sometimes it is the result of treatment for an overactive thyroid working too well, in which case the vet needs to adjust medication levels. PetMD further explains that besides observing symptoms of feline distress, diagnosis is a matter of running blood chemistry and urinalysis screenings to ascertain levels of the thyroid hormones T3 and T4. The vet can also be alerted to this potential problem by palpitating the thyroid gland in your cat's throat.
Correcting the problem is fairly simple, although it's a delicate balance between too little or too much hormonal supplementation. The vet may need to adjust treatment several times before the cat's T3 and T4 levels are just right. Once that balance is attained, the cat needs follow-ups every six months to one year.
Once T3 and T4 levels are correct, the cat recovers rapidly: any hair lost starts regrowing within a week, and extra weight comes off within the first two weeks. At the end of four weeks, the cat should be recovered from the hypothyroid symptoms and back to its normal self.