History: A 65 year old housewife complains of progressive weight gain of 40 poun

History: A 65 year old housewife complains of progressive weight gain of 40 pounds in 1 year, fatigue, dizziness, sluggish memory, slow speech, deepening of her voice, dry skin, constipation, and cold intolerance.
Physical examination: Vital signs: temperature 96.4oF, pulse 68/minute and regular, BP 108/60, weight 170 lbs, height 5 feet, puffy face, pale, cool, dry skin. The thyroid gland is not palpable, deep tendon reflex time is delayed.
Laboratory studies: CBC and differential WBC are normal. The serum T4 concentration is 3.4 ug/dl (N=4.5-12.5), the serum TSH is 0.9 uU/ml (N=0.2-3.5), and the serum cholesterol is 275 mg/dl (N<200). What is the likely diagnosis and what symptoms made you consider that diagnosis?  Which lab data supported the diagnosis?  Explain-Hypothalamic-Pituitary-Thyroid axis and interrelationship.