The authors have encountered multiple clinical cases of clinical hyperthyroidism in the guinea pig, which responded positively to clinical treatment. Hyperactive thyroids in the guinea pig appear to exist causing typical clinical signs. An early accurate diagnosis of this pathologic state is important in the clinical setting. One of the authors has encountered a few clinical cases of hypothyroidism in guinea pigs. Hypothyroidism appears to be a rare condition and has been described anecdotally in the German literature. Because of the rarity of hypothyroidism, the text focuses mainly on the guinea pig as a hyperthyroid case. A short description of the clinical presentation of the hypothyroid animal is included at the end of the text.