Hypothyroidism Flashcards
What does hypothyroidism result form
Insufficient secretion of thyroid hormones
What are the 3 different forms of hypothyroidism
Primary
Subclinical
Central
What is primary hypothyroidism characterised by
Low serum free thyroxine (T4) and high serum thyroid stimulating hormone (TSH) levels (due to a reduced negative feedback effect of T4 of TSH synthesis/ secretion)
What is subclinical hypothyroidism defined as
Normal serum free T4 and T3 levels and a high serum TSH. This reflects the sensitivity of tTSH secretion to very small decreases in thyroid hormone secretion
What is central hypothyroidism
Much less common
Results from reduced TSH secretion from the Anterior pituitary (secondary hypothyroidism) or reduced thyrotrophin-releasing hormone (TSH ) secretion from the hypothalamus (territory hypothyroidism)
How common is hypothyroidism
1 in 4000 of the population
What sex is hypothyroidism more common in
Females (5-8 times)
What are some of the congenital causes of hypothyroidism
Thyroid agenesis
Dysgenesis or inherited defects in thyroid hormone biosynthesis
What are some of the aquired causes of hypothyroidism
Chronic autoimmune (HASIMOTO’s) thyroiditis
Iatrogenic (drugs, thyroidectomy, radio iodine, neck radiotherapy)
Iodine deficiency / excess
Thyroiditis
Pituitary / hypothalamic damage (tumours, trauma, radiotherapy)
What causes Hashimoto’s
Cellular and antibody-mediated injury to the thyroid tissue
Patients with chronic autoimmune thyroiditis are more likely to have what ?
Personal or family history of other autoimmune conditions such as Assidosn’s disease and type 1 diabetes mellitus, vitiligo
How can some patients become hypothyroid
Following radio iodine therapy for the treatment of Graves’ disease
After subtotal thyroidectomy for the treatment of Graves’ disease
External irradiation of the neck
Iodine deficiency and excess
In postpartum and subacute thyroiitis
Describe the typical clinical presentation of hypothyroidism
Fatigue, lethargy, cold intolerance
Depression, goitre
Dry and course skin, pale / yellow tinge, oedematous
Course hair, hair loss, brittle nails
Bradycardia, exertional breathlessness, exacerbation of heart failure or angina
Hypercholestolaemia and hypertriglyceridaemia
Obstructive sleep apnoea
Constripaitonm, weight gain, ascites
Hyperprolactinaemia
Oligomenorrhoea/ ammenorrhoea, early abortion
Reduced libido, erectile dysfunction
Slowing of intellectual activities, movement, speech, impaired memory
Describe the symptoms in secondary hypothyroidism compared to those in primary hypothyroidism
Symptoms are usually milder and may be masked by symptoms of other hormone deficiencies e.g. hot flushes secondary to hypogonadism
What investigations should be carried out for suspected hypothyroidism
Measuring serum TSH and free T4
Hyperlipidaemia and hyponatraemia (in some patients)
Pituitary function tests and a MRI scan of the hypothalamus and pituitary for central hypothyroidism
What is the treatment for hypothyroidism
Lifelong treatment with synthetic levothyroxine
What are the objectives of treatment
Resolution of symptoms, a normalisation of TSH and a reduction in the size of the goitre in patients with Hashimoto’s thyroiditis
What is the dose of levothyroxine for younger patients (under 50)
50ug daily
Dose is increased by 25g until the TSH is in the normal range and as close to 1.0mU/L as possible
What is the average dose of levothyroxin in hypothyroid adults
100ug per day but the range varies from 50-200
How should levothyroxine be taken
On an empty stomach
What is the treatment for myxoedema coma
Intensive care unit Mechanical ventilation for respiratory failure
Bloods for culture, free T4, T3, TSH and cortisol before starting treatment
IV levothyroxine 300-500ug
IV hydrocortisone
Replace IV fluids and glucose appropriately
Correct hypoterhmia using a heating blanket
What is myxoedema coma
Myxedema coma is a state of decompensated hypothyroidism. A person may have lab values identical to a “normal” hypothyroid state, but a stressful event (such as an infection, myocardial infarction or stroke) precipitates the myxedema coma state, usually in the elderly.
If adrenal insufficiency or secondary hypothyroidism is suspected, what should be done prior to commencing treatment ?
why?
Short Synacthen test
it can cause Addisonian crisis in patients with untreated adrenal insufficiency
When should TSH be measured after starting oestrogen therapy
12 weeks
What is the dose of levothyroxine in older patients (over 50)
Older patients should start on just 25g daily as they may have ischaemic heart disease and levothyroxine increases myocardial oxygen demand