Different signs and symptoms of hypothyroidism can vary, it all depends on the range and severity of the thyroid problem, and this is classified as hypothyroidism. These issues can develop at a slow pace if not treated which, makes it difficult to notice anything being out of place. Usually, weight gain and fatigue will be brushed aside as nothing to worry about because mostly everyone deals with this sometime in their life. Hypothyroidism mostly affects women who are middle-aged or older.

Fatigue is one of the most obvious and first sign, even if you have gotten enough sleep. You are clearly lacking energy and suffering from brain fog most likely which impairs your mental clarity. Weight gain is a common sign; if you eat healthily and workout often and still sees weight gain then it’s most likely due to a thyroid problem.
Dry skin, even if it’s not due to the weather or the environment, can be a problem with thyroid problems. Depression/lack of motivation is another symptom to watch out for. If you are usually a happy and upbeat person, that is easily motivated by things but lately, you’ve been feeling depressed for no reason, and don’t feel like doing anything challenging, it can be a sign with something being wrong with your thyroid.
Memory problems can be another symptom, if you find yourself leaving reminder notes more often and usually you can remember things easily, that may alert you to check your thyroid for issues.
Slower heart rate, elevated blood cholesterol levels, constipation, muscle weakness/aches, tenderness, stiffness, swelling/pain in joints, thinning hair, bigger/irregular menstrual cycles for women, hoarseness, puffy face and an increase sensitivity to cold are all physical signs and symptoms to watch out for.

When left untreated these symptoms or signs will slowly become more severe. When the thyroid issues become advanced hypothyroidism (Myxedema) which is rare but can be life-threatening. When this happens the symptoms and signs are lower blood pressure, decreased breathing and body temperature, being unresponsive and can result in a coma. Hypothyroidism can occur in babies, even though older women are the most affected by it. Signs and symptoms in infants are different compared to older women who have it.

Jaundice which is yellowing of the skin and eyes are whiter than normal. This happens usually, because when the body re-uses old or damaged red blood cells.
Frequent choking
Puffiness appearance around the face
Trouble with feeding
Failure of growing and develop normally
Constipation problems
Weak muscle tone
More sleepy

When hypothyroidism is found in kids and teenagers they usually have the same signs and symptoms that appear in adults, but they can also experience:

Not having a lot of growth development (remaining short)
Delayed symptoms such as (permanent teeth, puberty and poor, mental development)

What causes this? When the thyroid doesn’t get enough hormones, the chemical balance reactions that go on in your body can get messed up and out of whack. What causes this can be a number of reasons such as autoimmune disease, different types of treatment for hyperthyroidism, radiation therapy, thyroid surgery and certain medications that may have side effects. It can also be caused by congenital disease, pituitary disorder, pregnancy and iodine deficiency less often.

Complications can occur if left untreated and can evolve into serious problems such as:

Goiter (can affect swallowing and breathing)
Heart problems (heart disease, heart failure, higher cholesterol levels, lower pumping ability to the heart)
Mental health problems (depression that can get worse over time, slower mental processing and functioning ability)
Peripheral neuropathy (causes damage to peripheral nerves, pain/numbness/tingling around the area it is affected can be a sign)
Myxedema (rare but life-threatening, intense cold intolerance, drowsiness/fatigue/lethargy, unconsciousness and possibly a coma)
Infertility (If you have autoimmune disorder infertility can be affected by it because of low levels of thyroid hormones)
Birth defects (More birth defects can be found in infants who comes from a mother who has untreated thyroid disease, intellectual and development problems can also occur along with physical and mental development.)

Treating hypothyroidism can vary somewhat such as going on a certain diet and avoiding consumption of certain foods, not taking certain supplements or changing when you take them during the treatment process because it will interfere with the thyroid hormone medication.

Always check and speak with your doctor before self-diagnosing. It is important to listen to your body and keep an eye on signs and symptoms of possible underlying issues. Speak with a healthcare professional if you think you may be experiencing these or any other abnormal symptoms.

Signs, symptoms and diagnosis information cited from: