People who did not live near the sea were liable to suffer from thyroid problems due to a lack of iodine in their diet - although who actually suffered was determined by hereditary factors as well as by a lack of iodine in salt and in drinking water - which would explain why Emma was fee from any symptoms.

Hypothyroidism symptoms include
nodules in the throat (if not goiter) that can make swallowing hard foods unpleasant;
low body temperature (feeling cold when others feel hot);
feeling fatigued, exhausted;
Feeling run down, sluggish, lethargic;
a hoarse voice - leading to a preference to talk softly;
pains, aches in joints, hands and feet;
carpal-tunnel syndrome;
depression;
restlessness;
moods change easily;
difficulty concentrating;
feelings of sadness;
forgetful, difficulty remembering things;
frequent infections, that last longer;
may have sleep apnea;
shortness of breath and tightness in the chest;
feel the need to yawn to get oxygen;
eyes feel sensitive to light;
eyes get jumpy/tics in eyes, causing feelings of vertigo and headaches;
strange feelings in neck or throat;
tinnitus (ringing in ears);
recurrent sinus infections;
lightheadedness;
Of course, hypochondriacs have many of the same symptoms.

People with hypothyroidism are also likely to be plump, or gain weight, and are not famous for their quick wits.
Women who suffer from hypothyroidism are more prone to miscarriage, and difficulties in conception and pregnancy - Isabella appears to be lucky in that respect, or perhaps a little sea bathing has set her (or rather, her children) up forever.