Endocrine Weight Loss Thyroid Function and Weight Loss 1 — Questions and Answers
Question 1: Which thyroid hormone is considered the most metabolically active form at the cellular level?
- T4 (thyroxine)
- T3 (triiodothyronine) (Correct answer)
- Reverse T3 (rT3)
- Thyroglobulin
Correct answer: T3 (triiodothyronine)
T3 (triiodothyronine) is 3–4 times more potent than T4 and is the primary hormone that binds thyroid hormone receptors in cell nuclei to regulate gene expression and metabolism.
Question 2: Subclinical hypothyroidism is defined as:
- TSH >10 mIU/L with normal free T4 and symptoms
- Elevated TSH with normal free T4, often asymptomatic (Correct answer)
- Normal TSH with low free T4
- Low TSH with low free T4
Correct answer: Elevated TSH with normal free T4, often asymptomatic
Subclinical hypothyroidism is characterized by a TSH above the upper reference limit (typically >4.5 mIU/L) with a normal free T4, and may present with subtle or no symptoms.
Question 3: Which autoantibody is most closely associated with Hashimoto's thyroiditis, the most common cause of hypothyroidism in the US?
- TSH receptor antibody (TRAb)
- Anti-thyroglobulin antibody only
- Anti-thyroid peroxidase antibody (anti-TPO) (Correct answer)
- Anti-smooth muscle antibody
Correct answer: Anti-thyroid peroxidase antibody (anti-TPO)
Anti-thyroid peroxidase (anti-TPO) antibodies are present in >90% of patients with Hashimoto's thyroiditis and are the hallmark autoimmune marker of this condition.
Question 4: A patient with hypothyroidism gains weight primarily because of:
- Increased fat synthesis stimulated by elevated TSH
- Accumulation of myxedema fluid and reduced metabolic rate leading to fat deposition (Correct answer)
- TSH-driven leptin resistance
- Decreased hepatic fat oxidation due to excess thyroglobulin
Correct answer: Accumulation of myxedema fluid and reduced metabolic rate leading to fat deposition
Hypothyroid weight gain results from reduced metabolic rate (lower thermogenesis) plus accumulation of glycosaminoglycan-rich interstitial fluid (myxedema), though the latter is not true fat.
Question 5: Which thyroid function test is considered the single best screening test for thyroid dysfunction?
- Free T4
- Free T3
- Thyroid-stimulating hormone (TSH) (Correct answer)
- Total T4
Correct answer: Thyroid-stimulating hormone (TSH)
TSH is the most sensitive indicator of thyroid status because it reflects pituitary integration of free thyroid hormone levels; even small changes in free T4/T3 cause large TSH changes.
Question 6: Levothyroxine (T4) replacement therapy restores metabolic rate and promotes weight loss in hypothyroid patients primarily through which mechanism?
- Directly stimulating adipose lipolysis via beta-adrenergic receptors
- Peripheral conversion to T3, which increases mitochondrial uncoupling and basal metabolic rate (Correct answer)
- Suppressing TSH and thereby reducing myxedema
- Enhancing insulin sensitivity via hepatic PPAR-alpha activation
Correct answer: Peripheral conversion to T3, which increases mitochondrial uncoupling and basal metabolic rate
Exogenous T4 is peripherally deiodinated to the active T3, which binds nuclear thyroid receptors to upregulate genes controlling mitochondrial biogenesis, uncoupling, and overall metabolic rate.
Endocrine Weight Loss Thyroid Function and Weight Loss 1
Which thyroid hormone is considered the most metabolically active form at the cellular level?